The United States believes North Korea has suffered some "significant" troop losses during combat alongside Russian forces against Ukraine, a White House official confirmed Monday, noting the troops were seen moving from the second lines on the battlefield to the front lines.

National Security Communications Adviser John Kirby made the remarks as Kyiv's intelligence authorities have said that some 200 Russian and North Korean soldiers were estimated to have been killed while fighting in combined units against Ukrainian forces.

"We do believe that they have suffered some significant losses, killed and wounded, but it's difficult for me to put an actual number on it. I would say, certainly in the realm of dozens, several dozens," Kirby said in an online press briefing.

"We've just started to see this movement of them from the second line to the front line. So, it's a fairly new development, and we might be able to have a little bit more granularity as days go on," he added.

It marks the first time that Washington publicly confirmed North Korean fatalities during Russia's war against Ukraine.

State Department spokesperson Matthew Miller also confirmed the North's fatalities, warning that should North Korean troops cross the border into Ukraine, it would mark "yet another escalation."

"In our view, the North Korean soldiers who were deployed to Kursk are already legitimate targets. They entered a war, and they are as such combatants that are legitimate targets for the Ukrainian military," Miller told a press briefing.

"We have seen North Korean soldiers who have been killed in action on the battlefield inside Russia, and if they were to cross the border into Ukraine, that would be yet another escalation by the government of Russia and also an escalation by the government of North Korea to send North Korean troops to prosecute a war of aggression against an independent sovereign nation inside that nation's borders," he added.

Miller also pointed out that there is "certainly more" that China can do to encourage Russia not to take escalatory action and other moves that it has taken.

Earlier in the day, Pentagon press secretary Maj. Gen. Pat Ryder said that the US assesses that North Korean troops have suffered casualties, including fatalities, during combat against Ukraine, according to Reuters.

"We do assess that North Korean soldiers have engaged in combat in Kursk. ... We do have indications that they have suffered casualties, both killed and wounded," Ryder was quoted by Reuters as telling reporters. Kursk is Russia's western front line.

Ryder noted that North Korean troops entered combat last week.

In a statement published Saturday, Ukraine's Defense Intelligence of Ukraine said that North Korean soldiers have been mobilized for assault operations as part of combined units of Russian marines and airborne troops.

The DIU claimed that North Korean troops suffered "irreversible losses" from Ukraine's attacks and noted that the language barrier is an issue for Russian and North Korean combined units in conducting coordinated operations.

The US has said that Pyongyang has sent more than 11,000 troops to support Russia in its war against Ukraine. The troop deployment has fueled concerns that it could further prolong the war and have security implications for both Europe and the Indo-Pacific. (Yonhap)