A London-based institute has claimed that it has identified Russian battle tanks inside eastern Ukraine, adding that the only way they would have made it across the Ukrainian border is if they were procured from the Russian army by pro-Moscow rebels. Russia has consistently denied supplying arms and military equipment to the region's separatists.

The International Institute for Strategic Studies said, according to a BBC report, that at least three Russian battle tanks reportedly entered eastern Ukraine at a time when Kiev has claimed that rebels on Tuesday began a new offensive led by Russia. A video that surfaced on Tuesday shows a convoy of tanks, of which at least three are said to be Russia-made T-72B1 Main battle tanks, moving through Sverdlovsk in Luhansk. 

Although the date of the video has not been confirmed, Joseph Dempsey, an IISS analyst, told BBC that the tanks are not known to have been exported or operated outside Russia, adding that the presence of the tanks in Ukraine supports the theory that Russia is arming separatists in the region. The report seeks to clarify an earlier theory that rebels had taken control of armored vehicles of the Ukrainian government after some of the tanks in the rebel ranks were identified as the T-63BV, which is mainly operated by the Ukrainian army. 

So far, Moscow has denied claims by Ukraine and Western nations that it is backing the rebels in Ukraine and is preparing to invade the country.

“We are now evidently seeing fighting between regular Russian and regular Ukrainian forces in Eastern Ukraine. There is a word for this,” Swedish foreign minister Carl Bidt, tweeted Thursday, accusing Russia of direct intervention in Ukraine.

Fighting, marked by the use of tanks, artillery and infantry units, continued on Wednesday in the port town of Novoazovsk, in the southern part of separatist-controlled territory, eight miles from the Russian border. Although the Ukrainian military has claimed that it took back control of the region, the mayor of the town reportedly said that pro-Moscow troops had moved into the town.

“These incursions indicate a Russian-directed counteroffensive is likely underway in Donetsk and Luhansk. Clearly, that is of deep concern to us,” Jen Psaki, the spokesperson for the U.S. Department of State, said in a briefing Wednesday, adding: “We’re also concerned by the Russian Government’s unwillingness to tell the truth even as its soldiers are found 30 miles inside Ukraine. Russia is sending its young men into Ukraine."

Here is the video of the tanks moving through Luhansk.