North Korean Dictator Kim Jong-Un has sent troops to Russia, the Pentagon confirmed Wednesday.

Defense Secretary Lloyd Austin confirmed the troop movement in a statement, saying it would be a "a very, very serious issue" if North Korean troops deploy to Ukraine. South Korean intelligence stated that some 3,000 troops were sent to Russia and are currently being trained on drones and other equipment.

"What exactly are they doing? Left to be seen. These are things that we need to sort out," Austin said.

Intelligence officials in both South Korea and Ukraine had previously stated that North Korean troops were being transferred to Russia.

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Russia and North Korea have denied the troop movements.

South Korean National Intelligence Service Director Cho Tae-yong told lawmakers that North Korea plans to deploy 10,000 troops to Russia by December, a number that lines up with claims from Ukrainian intelligence.

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So far, 3,000 troops have made the trip, but none have reportedly been deployed to battle.

South Korea said Tuesday that it may start sending weapons to Ukraine in reaction to the deployment. Officials said their response would come in phases linked to the depth of cooperation between Russia and North Korea.

NATO Secretary-General Mark Rutte has said it would be a "significant escalation" if North Korean troops fight alongside Russians in Ukraine.

North Korea and Russia, locked in separate confrontations with the West, have sharply boosted their cooperation in the past two years. The U.S., South Korea and their partners have accused North Korea of supplying artillery shells, missiles and other conventional arms to Russia to help fuel its war against Ukraine in return for economic and military assistance. In June, Kim and Russian President Vladimir Putin signed a pact stipulating mutual military assistance if either country is attacked.

Fox News' Greg Wehner and the Associated Press contributed to this report.