LONDON -- 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.

This is a developing story. Check back for updates.

ABC News' Chad Murray contributed to this report.