U.S. officials say they expect a Russian attack on Ukraine in the next few days that could involve a broad combination of jet fighters, tanks, ballistic missiles and cyberattacks, with the ultimate intention of rendering the country’s leadership powerless.

These officials said prospects for averting war now appear very dim, while adding that the Biden administration will continue to try to keep open the window for diplomacy.

The...

U.S. officials say they expect a Russian attack on Ukraine in the next few days that could involve a broad combination of jet fighters, tanks, ballistic missiles and cyberattacks, with the ultimate intention of rendering the country’s leadership powerless.

These officials said prospects for averting war now appear very dim, while adding that the Biden administration will continue to try to keep open the window for diplomacy.

The officials said Russian President Vladimir Putin has laid the groundwork in recent days through a series of false-flag operations—long predicted by U.S. and allied officials—that intended to make it look as if Ukraine has provoked Russia into a conflict, thus justifying the Russian invasion.

Russia has now amassed between 169,000 and 190,000 military personnel near Ukraine and in Crimea, up from a force of 100,000 on Jan. 30, Michael Carpenter, the U.S. ambassador to the Organization for Security and Cooperation in Europe, said in prepared remarks to a security conference on Friday.

“This is the most significant military mobilization in Europe since the World War II,” said Mr. Carpenter, who noted that the U.S. estimate includes Russian troops, Russian internal-security units and Russian-led forces in Donbas.

The Kremlin and Russia’s Ministry of Foreign Affairs didn’t immediately respond to a request for comment on Washington’s assessment.

Earlier in the day, Foreign Ministry spokeswoman Maria Zakharova told a state television program that Russia had told the U.S. Moscow had no intention of invading Ukraine.

Helicopters and troops at Lake Donuzlav, Crimea, Ukraine, earlier in February.

Photo: maxar technologiesEPA//Shutterstock

A satellite image shows a battle group departing from a vehicle park in Yelnya, Russia, north of the border with Ukraine, earlier in February.

Photo: Maxar Technologies/Associated Press

Ukrainian officials have said in recent weeks that the U.S. is exaggerating the imminence and potential scale of an attack.

Other Western officials say that the Russian buildup has grown to 110 battalion tactical groups, up from 83 such groups earlier this month, and that an attack could come at any time. They said Mr. Putin still has an array of military and diplomatic options, and it remains unclear which he might choose.

Secretary of State Antony Blinken also said Friday that Russia was behind the reports of mortar attacks and a broader assault against Russian citizens in the Donbas region of southeastern Ukraine, which the Kremlin is using to justify a potential military intervention.

“Everything we are seeing, including what you described in the last 24-48 hours, is part of a scenario that is already in play of creating false provocations, of then having to respond to those provocations and then ultimately committing new aggression against Ukraine,” Mr. Blinken said at the Munich Security Conference on Friday.

President Biden said Thursday that an attack appeared to be at hand.

“Every indication we have is they’re prepared to go into Ukraine, attack Ukraine,” Mr. Biden said. “My sense is this will happen within the next several days.”

Mr. Biden plans to speak on the Ukraine crisis at 4 p.m. EST following a call with allied leaders.

On Thursday, Mr. Blinken proposed a meeting with Sergei Lavrov, his Russian counterpart, next week in Europe that could lead to a summit of key leaders. Mr. Lavrov agreed to the meeting, but the U.S. said that it would only take place if Russia didn’t invade.

Among the Russian forces amassed near Ukraine are battalion tactical groups. Those units, which generally number about 700 to 800 troops each, are manned by professional soldiers instead of conscripts. Built around mechanized infantry or tank battalions, they are reinforced with artillery, air defenses, electronic warfare and other units.

Footage shows a kindergarten and a school in Kyiv-controlled towns of the Donbas region damaged by mortar shells, according to the Ukrainian army. Authorities in Russian-controlled areas said mortar attacks had also damaged several buildings in towns there. Photo: Aleksey Filippov/AFP/Getty Images The Wall Street Journal Interactive Edition

Earlier, the leaders of two Russian-controlled regions of eastern Ukraine urged civilians to evacuate to Russia immediately as Washington warned that Moscow is gearing up for an imminent military invasion of Ukraine and seeking an excuse to unleash hostilities.

The escalation came as shelling on the front line between Ukrainian and Russian-controlled parts of Donetsk and Luhansk regions, collectively known as Donbas, intensified on Friday and Kyiv said its troops have been ordered to exercise restraint in responding to artillery fire so as to not give Moscow a pretext to invade.

Denis Pushilin, the head of the Russian-controlled Donetsk People’s Republic, said in a televised address Friday that civilians in his statelet should immediately move to Russia because Ukrainian forces were about to invade the area, under Moscow’s control since 2014. “A temporary departure will protect life and health for you and your loved ones,” he said.

The head of the nearby Luhansk People’s Republic issued similar instructions, ordering able-bodied men to take up arms against what he also described as an imminent Ukrainian attack. Video footage on Russian news channels showed air-raid sirens going off in Donetsk on Friday afternoon.

Symbolic paper angels pay tribute to protesters killed during the uprising against pro-Russian President Viktor Yanukovych in 2014.

Photo: sergei supinsky/Agence France-Presse/Getty Images

Ukraine says it has no intention to retake Russian-held parts of Donetsk and Luhansk by force. “We categorically refute Russian disinformation reports on Ukraine’s alleged offensive operations,” Ukrainian Foreign Minister Dmytro Kuleba tweeted Friday. “Ukraine does not conduct or plan any such actions in the Donbas. We are fully committed to diplomatic conflict resolution only.”

Heavy artillery, tank and mortar exchanges picked up Friday morning along the cease-fire line separating the Ukrainian-controlled two-thirds of Donetsk and Luhansk from Russian-held areas, according to accounts from both sides. Violence in the regions had already increased Thursday, with shells hitting a kindergarten and a school in Ukrainian-controlled towns. There were no reports of fatalities.

Ukrainian and Western officials say the Kremlin, which has issued Russian passports to residents of the Russian-controlled parts of Donetsk and Luhansk and alleges that Kyiv is perpetrating a “genocide” there, is trying to goad the Ukrainian forces into hitting civilian targets. The same strategy was used in 2008, when Russian troops invaded the republic of Georgia, citing as an excuse the Georgian shelling of the capital of South Ossetia, a Moscow-controlled breakaway statelet.

Secretary of State Antony Blinken said Friday that Russia was behind the reports of mortar attacks against Russian citizens in Donbas.

Photo: Michael Probst/Associated Press

“They may be trying to force our hand in negotiations, or attempting to provoke response fire and use that as a pretext to start a full-scale invasion. There are several options here, and the South Ossetian option is very probable,” Serhii Haidai, the Ukrainian governor of Luhansk region, said in an interview Friday. Ukrainian forces, he added, have been given orders to hold their fire as much as possible. “They shoot back only when there is a direct threat to life, when they are being hit in the direct proximity to their own position,” Mr. Haidai said.

The commander of Ukrainian forces in Donetsk and Luhansk regions, Lt. Gen. Oleksandr Pavliuk, also said Russia is doing everything it can to provoke Ukraine into shelling civilian areas, so that it could back up its genocide allegations, according to a transcript published by the Defense Ministry. Some 14,000 people have died in the Donbas conflict since Russia fomented a separatist uprising there in 2014, sending weapons and troops across the border.

“Unfortunately, we are currently seeing an escalation of the situation in Donbas,” Mr. Putin said Friday after meeting his Belarusian counterpart.

Mr. Pushilin, a Russian citizen and a member of the ruling United Russia party, said he had secured an agreement of the Russian government to host Donetsk residents in the nearby Rostov region of Russia. Women, children and retired people will be the first to be evacuated, and offices, institutions and companies have been instructed to organize the departure of their staff, he said. The evacuation is beginning immediately, he said.

Mr. Pushilin said that in recent months authorities in Donetsk had observed a daily increase in the number of military personnel and lethal weapons of the Ukrainian army, including multiple rocket-launch systems, antitank missiles and air-defense systems. “Today their guns are aimed at civilians, at us and our children,” Mr. Pushilin said. “The enemy’s armed forces are in combat formations and are ready for the forceful capture of Donbas.”

Russian-installed Donetsk authorities earlier Friday said that they had intercepted a Ukrainian infiltration team allegedly attempting to attack a chemical plant in the city of Horlivka. Kyiv denied that such an incident occurred. Ukrainian and Western officials have long warned of Russian-installed authorities creating a false-flag incident they could pin on Kyiv.

While skirmishes along the front lines in Donbas have occurred repeatedly since major combat ended in 2015, the upsurge in shelling that started on Thursday is anomalous, Ukrainian Defense Minister Oleksii Reznikov said. From the beginning of the year and until Feb. 16, Russian-backed forces fired 107 times on Ukrainian military positions along the cease-fire line, he said, and only 22 of these incidents involved heavy artillery, tanks, antitank missiles and other weapons prohibited under the cease-fire agreements.

On Thursday, there were 60 shelling incidents, out of which 43 used prohibited weapons, with 30 Ukrainian-held towns and villages across the front line getting hit, Mr. Reznikov said. There were 33 more incidents by 11 a.m. on Friday, according to the Ukrainian military. “Our task is not to do what the Russian Federation is pushing us to do,” Mr. Reznikov told a parliament hearing Friday. “Our task is to respond, but to keep a cool head.”

Russian troops are moving closer to Ukraine’s border, dispersing along it, and increasing their logistical capacities, U.S. Defense Secretary Lloyd Austin told reporters during a visit to Warsaw on Friday. “We see more forces moving into that region, that border region.”

Poland has been a close U.S. ally against Moscow, and is hosting 4,700 U.S. troops being rotated onto Polish territory in response to the Russian escalation around Ukraine. “We all do believe that through being decisive, and through the decisive policy of the free world, especially the United States, there will be no war,” Polish Defense Minister Mariusz Blaszczak said. “We do still hope and believe that there will be no conflict, due to the unity of the free world.”

Moscow denies that it intends to invade Ukraine but says that it is duty-bound to protect Russians and Russian speakers in Donbas. It also says it can’t accept Ukraine joining the North Atlantic Treaty Organization.

Ukrainian President Volodymyr Zelensky, who toured the front line in Donbas on Wednesday and Thursday, was slated to fly Saturday to Munich for a series of meetings at the international security conference there, including with Vice President Kamala Harris. Russia isn’t attending this year.

Mobile-phone service stopped functioning for some subscribers in many Ukrainian-controlled parts of Luhansk and Donetsk on Thursday night. Ukrainian officials said Friday that operations of the local Vodafone affiliate were disrupted by Russian-backed saboteurs who destroyed network nodes. While connection was restored hours later, attempts to sabotage communications facilities of all the networks are likely to continue in coming days, they said.

Corrections & Amplifications
The Ukrainian foreign minister’s name is Dmytro Kuleba. An earlier version of this article misspelled his name as Dmytro Kubela. (Corrected on Feb. 18.)

Write to Gordon Lubold at Gordon.Lubold@wsj.com, Michael R. Gordon at michael.gordon@wsj.com, Yaroslav Trofimov at yaroslav.trofimov@wsj.com and Ann M. Simmons at ann.simmons@wsj.com