Moldova said it was placing its security forces on alert Tuesday following a series of explosions in Transnistria, a breakaway pro-Russian enclave that has stirred concern over the role that some 1,500 Russian troops stationed there could play in shoring up Moscow’s military campaign in neighboring Ukraine.

Authorities in the separatist region alleged that three separate terrorist attacks on Monday and Tuesday had targeted a military base, two Soviet-era radio towers broadcasting Russian-language stations and the headquarters of the state security service in Tiraspol. No casualties were reported.

Moldova has been on edge since the Russian invasion of Ukraine, which put the country, a small impoverished former Soviet state, on the edge of an active war zone. Transnistria, a narrow strip of land bordering Ukraine, was carved out of Moldova in the country’s 1992 civil war and is held by pro-Russian separatists.

‘The danger has reached real levels. Explosions can be heard in the country,’ Vadim Krasnoselsky, the president of Transnistria, wrote on Telegram.

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

Moscow has military forces—which it describes as peacekeepers—and a base there. A large part of the population of 350,000 have Russian citizenship following a campaign by Moscow to secure passports for them. Only a handful of authorities around the world recognize the independence of Transnistria.

The government of Moldovan President Maia Sandu said Tuesday that the explosions were a provocation aimed at creating tension between the enclave and the rest of Moldova.

“Our analysis shows that there’s tension between various forces inside Transnistria who are interested in destabilizing the situation,” Ms. Sandu told reporters after the security council meeting, according to an official press release.

Following the explosions, authorities in Transnistria raised the terrorism threat to the highest level, increased the combat readiness of security forces and declared a state of emergency, with military checkpoints installed near city limits.

“The danger has reached real levels. Explosions can be heard in the country,” Vadim Krasnoselsky, the president of Transnistria, wrote on Telegram. “Naturally, every challenge will be met with an adequate response.”

Ms. Sandu said the security council had recommended improving the combat readiness of security forces, increasing the number of patrols and checks near Moldova’s border with Transnistria and monitoring critical infrastructure more closely.

“We denounce any provocations or efforts to draw the Republic of Moldova into acts that can endanger peace in the country,” she said.

The headquarters of Transnistria’s state security service in Tiraspol after the blasts, in a photo issued by the breakaway region’s interior ministry.

Photo: /Associated Press

In early April, the General Staff of Ukraine’s Armed Forces said that Russia was conducting combat readiness checks inside Transnistria but didn’t say a strike on Ukraine from Transnistria was imminent.

On April 23, the deputy commander of Russia’s Central Military District, Col. Gen. Rustam Minnekayev,

said that Moscow sought control over all of southern Ukraine in order to secure a land corridor to Crimea, the Ukrainian peninsula that Moscow annexed in 2014, and to Transnistria, where he said Russian speakers suffer from discrimination.

Moldova slammed the statement as “not only unacceptable but also unfounded” and warned that it risked exacerbating tensions.

It is unclear whether Mr. Minnekayev’s statement reflects official Russian military policy or what the capacity is for Russian forces, already seriously stretched in their stuttering campaign to capture swaths of Ukraine, to stage a military operation from Transnistria.

On Monday, Russian Deputy Foreign Minister Andrei Rudenko said he doesn’t see risks for Russian citizens in Transnistria, countering Mr. Minnekayev’s earlier statement. The following day, Mr. Rudenko said Russia hopes to avoid being drawn into a conflict over Transnistria.

The Kremlin on Tuesday said it was following the situation in Transnistria closely. “Obviously the news coming from there is causing concern,” said President Vladimir Putin’s spokesman, Dmitry Peskov.

The coat of arms of Transnistria, Moldova’s pro-Russian breakaway region along the border with Ukraine.

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

The self-proclaimed republic lives in its own information space, with residents consuming Russian state TV reports and pro-Russian local media that advance a narrative at odds with that circulating in Moldova proper, where Ms. Sandu has sought to bring the country closer to the European Union and formally applied for membership on March 3.

But the separatist administration in Tiraspol has been reluctant to get involved in the war in Ukraine, said Cristian Vlas, a Moldovan political analyst based in Bucharest.

Transnistria has traditionally profited from stable trade relations with successive Ukrainian governments, Mr. Vlas said, with steady food imports coming in from Ukraine and an active contraband trade through the port of Odessa on the Black Sea.

Write to Matthew Luxmoore at Matthew.Luxmoore@wsj.com