The Pentagon estimates there are now "at least 10,000" North Korean forces inside Russia's Kursk Oblast, and Maj. Gen. Pat Ryder, the Pentagon's press secretary, said that "those numbers could go up."
Ryder said that overall there could be closer to 11,000 to 12,000 North Korean troops inside of Russia. Ryder said the U.S. fully expects the remaining troops still in eastern Russia to make their way to Kursk.
"We've seen the press reports about alleged combat ops. We're looking into those, but at this point, cannot corroborate those reports," said Ryder, who made the comments at a press briefing on Monday.
NATO had confirmed last month that North Korean troops have been deployed alongside their Russian counterparts in the Kursk region, the area within Russia where Ukraine has been waging an assault.
MORE: North Korean troops are already in Russia, US confirms"The deployment of North Korean troops represents: one, a significant escalation in the DPRK ongoing involvement in Russia's illegal war," NATO Secretary General Mark Rutte said, using the acronym of the country's official name -- the Democratic People's Republic of Korea.
North Korea has denied the reports of its forces being active in Russia. The country's foreign minister, Choe Son-hui, traveled to Moscow last month on an official visit.
Ryder also said he could not corroborate that there are small numbers of North Korean engineers inside Ukraine.
"But as you heard Secretary [of Defense Lloyd] Austin say last week, should these troops engage in combat support operations against Ukraine, they would become legitimate military targets," he said.
Ryder said it remained unclear exactly how the North Korean troops would be employed by Russia and referred back to last week's comments that they are receiving training in drone operations, artillery, and infantry.
"We would fully expect that the Ukrainians would do what they need to do to defend themselves and their personnel," he said.
"In so much is that these are potentially forces that are coming in to replace the massive numbers of losses that Russia is experiencing," said Ryder. "I think that's probably a fair assessment, and I certainly would not want to be a North Korean soldier, right?"
Moving about 10,000 additional troops into a battlezone may sound like a large movement, but Ryder did not have any information about what percentage of troops those North Korean soldiers might make up on the Russian side of the battle.
"I don't even have a ballpark number, other than to say, broadly speaking, what we saw in the past was essentially a conglomeration of various units on the Russian side, to include territorial defense forces attempting to push the Ukrainians back," said Ryder.
But Ryder said that the addition of the 10,000 to 12,000 North Korean forces "could be significant" from a combat capability, "but a lot of that will depend on how those forces are employed, how they're integrated into the Russian command and control."
Ryder said that if Ukrainian forces continue to demonstrate their willingness to fight they'll be able to hold onto the ground they've gained in Kursk.
ABC News' David Brennan contributed to this report.