The United States on Wednesday said North Korean troops have been deployed to Russia, raising the possibility they will be sent to fight in Russia's war on 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 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 dangerous 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, with another 10,000 due by December. Ukraine seconded those claims.

Moscow and the North Korean government in Pyongyang have previously denied South Korea's assertions about North Korean troops being prepared for fighting in Ukraine.

Russia and North Korea have expanded their military ties in recent months as part of a growing anti-U.S. partnership. Since Russia's full invasion of Ukraine in February 2022, North Korea has expressed support. And last June, the two nuclear-armed countries signed a mutual defense agreement.

Active participation by North Korea in the Ukraine war threatens to widen it beyond eastern Europe, analysts said. Russian President Vladimir Putin would feel indebted to North Korea's Kim Jong Un, and South Korea would feel threatened. Reports from Seoul say the government there is considering sending lethal weapons to Ukraine.

North Korea has previously supplied weapons to Russia, which has trained North Korean fighter pilots.

Experts said the extent of North Korea's possible role in Ukraine would depend on what type of troops it might supply. If the troops in Russia are engineers and technicians, they may be dispatched to Ukraine to observe how their weapon systems are performing.

But if they are special forces or combat troops, as South Korea says, "that is a completely different dimension to the war," said Duyeon Kim, senior fellow and Korea expert at the Center for a New American Security, a Washington think tank.

It would be the first time North Korean troops would be participating on the ground in a "hot war" since the Korean War three-quarters of a century ago, she said.

"Every troop that they have, whether directly involved in conflict or not, frees up another Russian soldier to participate in the lethal dimensions of this ongoing conflict," said Sydney Seiler, a former U.S. intelligence official specializing in the Korean peninsula.

Kim and Seiler spoke before Austin made his comments on Wednesday, in a podcast sponsored by the Center for Strategic and International Studies.

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