Defense Secretary Lloyd Austin said in Germany that the United States is investigating the cause of the explosions and “not really sure what that is all about”.

‘Certainly,’ he said, ‘we don’t want to see an overflow. It is important to ensure that we do everything we can to ensure that Ukraine is successful. And that’s the best way to go about it.”

Pentagon spokesman John Kirby told CNN in an interview earlier Tuesday that it was too early “to know exactly what happened here, who is responsible.”

“We are monitoring this as closely as possible,” he told the news channel.

A Russian military commander said last week that one of the goals was to create a corridor through southern Ukraine to Transnistria — a strip of land with a population of nearly 500,000 that is supported by Moscow and housing Russian troops. The region, which broke away after the collapse of the Soviet Union and sparked conflict in the early 1990s, is not recognized as independent by any country, but operates separately from Moldova.

It was unclear whether the commander’s comments about Transnistria reflected official policies, though Ukraine portrayed them as evidence of the Kremlin’s ambitions beyond its borders and Moldova had summoned the Russian ambassador to express “deep concern”.

No one immediately claimed responsibility for the explosion. A Moldovan government agency on Monday warned of possible attempts to create “pretexts to pressure the security situation in the Transnistria region”, and Ukraine’s defense ministry called the explosions a “planned provocation by Russian special services”.

Russia’s foreign ministry said on Tuesday the country wanted to avoid a scenario in which it “will have to intervene in the conflict in Transnistria”, in a statement published by Russia’s RIA news agency that described the news of explosions as “alarming”. The head of pro-Moscow separatist forces fighting to extend their hold in eastern Ukraine, Denis Pushilin, said the latest incidents in Transnistria would require “the continuation” of Russian military operations.

What is Transnistria and will Russia advance to Moldova?

The head of Transnistria, Vadim Krasnoselsky, accused Ukraine on Tuesday of “traces of terrorist attacks,” according to the Russian news agency Tass.

Moldovan President Maia Sandu accused factions in the separatist region of trying to “destabilize the situation”.

The Washington Post was unable to independently verify any party’s claims.

In Transnistria, the interior ministry said explosions in the village of Mayak damaged two antennas transmitting Russia’s radio on Tuesday morning. It released photos of the collapsed towers a day after it said several blasts had hit the State Security Ministry building in the capital, Tiraspol, and preliminary information suggested bullets had been fired from a rocket-propelled grenade launcher.

No injuries were reported, the ministry said. The office of the Transnistria leader on Tuesday reported a third incident, an attack on a military unit near the village of Parcani, without giving further details.

As the region raised its security threat level, Moscow said it was “watching Transnistria very closely”. Kremlin spokesman Dmitry Peskov added that there were no plans for contact between Russian President Vladimir Putin and his Moldovan counterpart, Sandu. “Of course the news of that is worrying,” he said.

Dan Lamothe contributed to this report.