North Korean troops are already in Russia, Lloyd Austin confirms

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

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

Ukraine and South Korea have warned that North Korean soldiers have 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.

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 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."

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.

This is a developing story. Check back for updates.

ABC News' Chad Murray contributed to this report.