KYIV, April 25 – Several explosions believed caused by rocket-propelled grenades hit the territory’s Ministry of State Security in Transdnistria, a Russian-controlled breakaway region of Moldova that borders Ukraine in south-west, officials said.

Transdnistria, a strip of land with about 470,000 people, harbors 1,500 Russian troops and it may pose a military threat to Ukraine if Russian forces try to capture Odesa.

There was no immediate claim of responsibility or reports of injuries.

Moldova’s Foreign Ministry said “the aim of today’s incident is to create pretexts for straining the security situation in the Transnistrian region.” The U.S. warned previously that Russia may launch “false-flag” attacks against its own side to create a pretext for invading other nations, The Associated Press reported.

Last week, Rustam Minnekayev, a Russian military commander, said the Kremlin wants full control of southern Ukraine, which he said would open the way to Transnistria.

Meanwhile, an estimated 2,000 Ukrainian troops holed up in a steel plant in the strategic southern port city of Mariupol are tying down Russian forces and apparently keeping them from being added to the offensive elsewhere in the Donbas. Over the weekend, Russian forces launched new airstrikes on the Azovstal plant to try to dislodge the holdouts.

Some 1,000 civilians were also said to be taking shelter at the steelworks, and the Russian military pledged to open a humanitarian corridor Monday for them to leave.

The Russian offer was met with skepticism by Ukraine. Deputy Prime Minister Iryna Vereshchuk said on the Telegram messaging app that Ukraine does not consider the route safe and added that Russia had breached agreements on similar evacuation routes before. She called on the United Nations to oversee an evacuation.

The city council and mayor of Mariupol said a new mass grave has been identified about 10 kilometers (6 miles) north of the city. Mayor Vadym Boychenko said authorities were trying to estimate the number of victims. It was at least the third new mass grave discovered in Russian-controlled areas near Mariupol in the last week.

Mariupol has been gutted by bombardment and fierce street fighting over the past two months. In addition to freeing up Russian troops, the capture of the city would deprive Ukraine of a vital port and allow Moscow to establish a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014. (ap/ez)