North Korean troops have deployed to Russia and may join the fight against Ukraine, U.S. officials said Wednesday.

The confirmation from U.S. officials came after South Korea and Ukraine in recent days have expressed alarm at the North Korea troop movements. South Korean lawmakers have said that 3,000 North Korean troops have left to Russia so far, and another 7,000 are expected to go, according to NBC News.

“We assess that between early- to mid-October, North Korea moved at least 3,000 soldiers into eastern Russia,” National Security Council spokesman John Kirby said, according to CNN. “We assess that these soldiers traveled by ship from the Wonsan area in North Korea to Vladivostok, Russia. … We do not yet know whether these soldiers will enter into combat alongside the Russian military, but this is certainly a highly concerning probability. After completing training, these soldiers could travel to western Russia and then engage in combat against the Ukrainian military.”

Kirby’s comments followed an initial confirmation of U.S. intelligence from Defense Secretary Lloyd Austin, who spoke to reporters in Rome on Wednesday.

“We are seeing evidence that there are North Korean troops that have gone to Russia,” Austin said. “What, exactly, they’re doing is left to be seen.”

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If North Korea joins the war in Ukraine, the consequences could be severe and widespread, the Defense secretary said.

“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, and it will have impacts not only on in Europe, it will also impact things in the Indo Pacific as well,” he said.

The troop movements have caused unease within NATO, which has begun discussions on how the alliance will respond should North Korea join the war, according to a spokeswoman.

“Allies have confirmed evidence of a DPRK troop deployment to Russia. If these troops are destined to fight in Ukraine, it would mark a significant escalation in North Korea’s support for Russia’s illegal war and yet another sign of Russia’s significant losses on the front lines,” NATO spokeswoman Farah Dakhlallah told Politico Europe in a statement.

“We are actively consulting within the Alliance on this matter, and the North Atlantic Council will receive a briefing from the Republic of Korea and further discuss this matter soon,” she added.

North Korea and Russia have strengthened their ties in recent years. In June, North Korean leader Kim Jong Un and Russian President Vladimir Putin signed an agreement to provide “mutual assistance in the event of aggression against one of the participants.”