LONDON– NATO confirmed on Monday that North Korean troops had been deployed to fight alongside their Russian counterparts in the Kursk region, the area inside Russia where Ukraine has carried out an attack.

“The deployment of North Korean troops represents, first, a significant escalation of the DPRK's ongoing involvement in Russia's illegal war,” said NATO Secretary General Mark Rutte, using the acronym of the country's official name – Democratic People's Republic of Korea.

“Secondly, another violation of UN Security Council resolutions. And thirdly, a dangerous expansion of the Russian war,” he added.

He called on Russia and North Korea to “immediately stop these actions.”

North Korea has denied reports that its armed forces have deployed to Russia or Ukraine.

“My delegation sees no need to comment on such baseless, stereotypical rumors,” a North Korean representative to the United Nations said during a General Assembly session last week, as quoted by South Korea's Yonhap News Agency.

Kremlin spokesman Dmitry Peskov has repeatedly dismissed concerns about growing bilateral ties. “This cooperation is not directed against third countries,” he said last week.

Foreign Ministry spokeswoman Maria Zakharova appeared to blame South Korea for the development, saying at a briefing last week that Seoul “should not have played along with the Kiev regime.”

South Korea has provided direct humanitarian aid to Kyiv, but no weapons. Earlier this month, Seoul said North Korean involvement in Ukraine posed a “serious security threat,” adding that it would “respond by mobilizing all available assets in cooperation with the international community.”

Rutte's confirmation on behalf of NATO followed US Defense Secretary Lloyd Austin's announcement last week that the US had evidence that Pyongyang's forces were already stationed in Russia.

“This is a very, very serious problem and it will have implications not only for Europe but also for things in the Indo-Pacific,” Austin warned during a visit to Rome, Italy.

“Exactly what they do” remains to be seen, Austin told reporters. But the defense minister said there was “certainly” a “strengthened, for lack of a better term, relationship between Russia and the Democratic People's Republic of Korea.”

Austin noted that Pyongyang has already provided Russia with “weapons and ammunition and this is a next step.”

White House national security spokesman John Kirby told reporters last week that U.S. intelligence estimated North Korea had moved at least 3,000 troops to eastern Russia in the first half of October.

The troops are expected to undergo “basic combat training” at several training areas in the region, he said.

Kirby said it was unclear what Russia would give North Korea in return for its troops.

“We know that Mr. Putin managed to buy North Korean artillery,” Kirby said. “He was able to obtain North Korean ballistic missiles, which he used against Ukraine. And in return, we have at least experienced technology exchanges with North Korea.”

Both Austin and Kirby said the deployment of Pyongyang's soldiers on the battlefield was a sign of Moscow's military strain.

“You’ve heard me talk about the significant losses he’s suffered over the last two and a half years,” Austin said. “This is an indication that he may be in even more trouble than most people realize.”

Both South Korea and Ukraine expressed concerns about the advance of North Korean troops into Russia before the US and NATO confirmed their presence there.

Ukrainian President Volodymyr Zelensky warned earlier this month that Kiev had “clear data” showing that North Korean personnel were intervening in the war.

“A new threat has emerged – the evil alliance between Russia and North Korea,” Zelensky said in a video statement posted on social media. “These are not only production workers, but also military personnel,” the president said. “We expect an appropriate and fair response from our partners in this matter.”

“If the world is silent now and we are confronted with North Korean soldiers on the front lines as regularly as we defend them against drones, it will not benefit anyone in this world and will only prolong this war,” Zelensky said.

South Korea's National Intelligence Service told lawmakers last week that about 3,000 North Korean soldiers were believed to have been sent to Russia so far and that the total was expected to reach 10,000 by December.

Speaking to reporters, opposition politician Park Sun-won told reporters that NIS assumed that Russian instructors would expect casualties among the new arrivals, but they were described as being in good physical and mental shape. According to the Russians, North Korean troops lack an understanding of certain elements of modern warfare, including drone strikes, Park said.

The NIS also told the briefing that there was evidence that North Korean authorities were trying to control and manage the families of soldiers sent to Russia. The measures included isolating soldiers' families and even relocating them, the NIS said.