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At least 3,000 North Korean soldiers have arrived in eastern Russia this month, the White House said on Wednesday, and while it is unclear exactly what they will do, there is a “highly concerning probability” that they will join the fight against Ukraine .

“We estimate that North Korea deployed at least 3,000 troops to eastern Russia between early and mid-October,” said John Kirby, spokesman for the National Security Council. “We believe that these soldiers traveled by ship from the Wonsan region of North Korea to Vladivostok, Russia. … We don’t yet know whether these soldiers will go into battle alongside the Russian military, but that is certainly a highly worrying probability. After completing their training, these soldiers could travel to western Russia and fight against the Ukrainian military.”

Earlier Wednesday, Defense Secretary Lloyd Austin became the first senior U.S. official to confirm on record that North Korea had sent troops to Russia, as North Korea and Russia have forged increasingly friendly ties since Moscow’s invasion of Ukraine.

“We are seeing evidence that there are North Korean troops that have gone to Russia,” Austin told reporters traveling with him in Rome on Wednesday. “What exactly they do remains to be seen.”

The U.S. does not believe North Korean troops have reached Ukraine, but the movements have raised deep concerns that the conflict may be seriously escalating. Austin said the U.S. is still trying to figure out what role the North Koreans will play and whether they intend to travel to Ukraine.

“If they are fellow warriors and intend to take part in this war on behalf of Russia, that is a very, very serious problem and it will have implications not only in Europe but also in things in India.” Also in the Pacific,” Austin said.

Kirby said on Wednesday that the US had informed the Ukrainian government and was in close coordination with allies and partners.

A senior administration official said earlier Wednesday that the training of North Korean soldiers and the possible preparation of their deployment to Ukraine was a sign of serious desperation on Russia’s part.

In Rome, Austin said Putin “may be in more trouble than most people realize.” Kirby added that turning to North Korea would be “a sign of weakness, not strength, for the Kremlin.” to obtain workers. Kirby also said the move was a violation of U.N. Security Council resolutions.

When asked what North Korea would get in return for helping Russia with manpower, Austin said the U.S. would also have to clarify that.

In recent months, Moscow and Pyongyang have deepened their anti-American military partnership, and the growing alliance has worried officials in Kiev and Washington.

“I can tell you one thing, though,” Kirby said Wednesday. “If they are used to fight Ukraine, they are fair game. These are fair goals.”

Ukrainian President Volodymyr Zelensky has repeatedly warned that North Korean troops are joining the war on Russia’s behalf, telling a NATO summit last week that “10,000” soldiers and technical personnel were being prepared.

A Ukrainian intelligence source previously told CNN that a small number of North Koreans have been working with the Russian military, mainly to help with technology and share information about the use of North Korean munitions.

Meanwhile, South Korea’s National Intelligence Service said on Friday that North Korea had sent 1,500 soldiers, including special forces, to Russia for training.

This story has been updated with additional developments.

CNN’s Kevin Liptak and MJ Lee contributed to this report.