The United States on Wednesday said North Korean troops have been deployed to Russia, raising the possibility they will be sent to fight in Ukraine — an assertion made by South Korea and the Ukrainian government in Kyiv.

The move may show that Moscow’s shortage of manpower amid exorbitant casualties is worse “than most people realize,” Secretary of Defense Lloyd J. Austin III said.

“We are seeing evidence that that there are [North Korean] troops that have gone to Russia,” Austin told reporters traveling with him in Rome. “What, exactly, they’re doing is left to be seen.”

It was the first official U.S. confirmation of reports from South Korean intelligence agencies that thousands of North Korean troops recently showed up in eastern Russia to undergo training. Austin said the U.S. is still trying to determine whether they will be deployed into Ukraine. If so, he warned, it would be a significant escalation.

“If they’re a co-belligerent, their intention is to participate in this war on Russia’s behalf, that is a very, very serious issue,” Austin said.

South Korea has said 3,000 North Korean troops are already in Russia and receiving training. Ukraine seconded those claims.

Moscow and the North Korean government in Pyongyang have expanded their military ties in recent months as part of an anti-U.S. partnership. Both countries previously denied South Korea’s assertions about North Korean troops being prepared for fighting in Ukraine.