North Korean troops are already in Russia, US confirms

The U.S. has evidence that North Korean troops are in Russia, Defense Secretary Lloyd Austin confirmed Wednesday.

"What exactly they're doing" remains to be seen, Austin told journalists while in Rome. "These are things that we need to sort out."

Ukraine and South Korea have said North Korean soldiers traveled to Russia for training ahead of planned deployment to fight on battlefields in eastern Ukraine and western Russia.

Austin said Wednesday that the U.S. would "continue to pull this thread" to establish whether Pyongyang can be considered a co-belligerent in the conflict.

"That is a very, very serious issue and it will have impacts not only in Europe, it will also impact things in the Indo-Pacific as well," Austin warned.

White House national security spokesperson John Kirby told reporters during a briefing Wednesday that U.S. intelligence has assessed that North Korea moved at least 3,000 soldiers into eastern Russia between early to mid-October, where he said they are believed to be currently undergoing a "basic kind of combat training" at multiple military training sites.

Austin said there is "certainly" a "strengthened relationship, for lack of a better term, between Russia and DPRK," using the acronym of the country's official name -- the Democratic People's Republic of Korea.

Pyongyang, he added, provides "arms and munitions to Russia and this is a next step."

The development may indicate resource strain on Russian President Vladimir Putin, Austin added.

"You've heard me talk about the significant casualties that he has experienced over the last two-and-a-half years," he said. "This is an indication that he may be even in more trouble than most people realize."

If the soldiers join the war, Kirby said it would be a "dramatic move" and shows Russia's "desperation" after suffering "extraordinary casualties" on the battlefield.

The U.S. hasn't determined if North Korean leader Kim Jong Un has received anything in return for sending these soldiers, according to Kirby, who also mentioned the growing defense relationship between North Korea and Russia.

"We know Mr. Putin has been able to purchase North Korean artillery. He's been able to get North Korean ballistic missiles, which he has used against Ukraine. And in return, we have seen, at the very least, some technology sharing with North Korea," Kirby said.

Kirby said the U.S. is consulting with allies on "how we might respond."

North Korea has denied the reports of its forces being active in Russia or Ukraine.

"My delegation does not feel any need for comment on such groundless stereotyped rumors," a North Korean representative to the United Nations said during a U.N. General Assembly session this week, as quoted by South Korea's Yonhap news agency.

Kremlin spokesperson Dmitry Peskov, meanwhile, said earlier this week of the reports, "There is a lot of contradictory information, and that is probably how it should be treated," describing North Korea as a close neighbor and partner.

"This should not cause anyone any concern, because this cooperation is not directed against third countries," Peskov added.

South Korea's National Intelligence Service told lawmakers at a Wednesday closed-door briefing that around 3,000 North Korean soldiers are believed to have been sent to Russia so far, with some 10,000 expected to be deployed by December.

After the briefing, opposition politician Park Sun-won told reporters that NIS had information indicating that "Russian instructors believe that North Korean soldiers are fit both physically and mentally, but they lack the understanding of modern warfare, such as drone attacks." The Russian instructors, the lawmaker added, "expect multiple casualties among the North Koreans."

The NIS told the politicians that the Russian military is recruiting a large number of Korean language interpreters and training North Korean soldiers on how to use modern military equipment and drones.

The NIS also said it had indications that North Korean authorities are taking measures to control and manage the families of the deployed soldiers, including by isolating and relocating them to certain locations.

Russian Foreign Ministry spokesperson Maria Zakharova, meanwhile, said at a Wednesday briefing that Seoul "should not have played along with the Kyiv regime."

Zakharova urged South Korea "not to fight in anti-Russian hysteria, but to think about the consequences for the security of South Korea, which could lead to the entry of the Republic of Korea into the Ukrainian conflict, if such decisions are made."

"The Russian Federation will really react harshly to any steps that may pose a threat to the security of the country, its citizens, wherever its citizens may be. These measures can be very tangible," she said, expressing hope "for the prudence and common sense of the authorities in Seoul."

In what appeared to be a message to the North Koreans, Chinese President Xi Jinping urged third countries to avoid getting involved in Ukraine during remarks at the BRICS Summit in Russia on Wednesday.

"In the context of the prolongation of the Ukrainian crisis, China, Brazil and the countries of the Global South have created a group of friends of peace," he said.

"It is important, based on the principles of preventing the spillover of conflicts, its spread to third countries and avoiding escalation of hostilities and refusing to add fuel to the fire, to promote an early de-escalation of the situation," he added.

ABC News' Chad Murray, Will Gretsky, Joohee Cho and Justin Gomez contributed to this report.