North Korea has sent around 10,000 troops to Russia to fight in Ukraine, according to the Pentagon.

“We have seen reports from over the weekend indicating that DPRK soldiers have started arriving in western Russia,” Pentagon spokeswoman Sabrina Singh said Monday. “We believe that the DPRK has sent around 10,000 soldiers in total to train in eastern Russia that will probably augment Russian forces near Ukraine over the next several weeks.”

U.S. officials said the new infusion of troops presents an escalation in the war that was sparked nearly three years ago by Vladimir Putin’s invasion into Ukraine.

“A portion of those soldiers have already moved closer to Ukraine, and we are increasingly concerned that Russia intends to use these soldiers in combat or to support combat operations against Ukrainian forces in Russia’s Kursk Oblast near the border with Ukraine,” Singh said.

The Pentagon reassured reporters that the development would not deter American efforts to defend Ukraine. “If we see DPRK troops moving in towards the front lines…they are co-belligerents in the war,” the spokeswoman added.

Russia has been forced to rely on other anti-Western powers during its campaign, notably Iran. The Islamic Republic remains a valuable source of drone production for Moscow’s offensive.

Earlier on Monday, NATO secretary-general Mark Rutte confirmed in a statement that North Korean troops had been sent to Russia. Rutte said military units had been deployed to the Kursk region — the site of a strong Ukrainian incursion that the Kremlin has struggled to combat.

“Pyongyang has already supplied Russia with millions of rounds of ammunition and ballistic missiles that are fueling a major conflict in the heart of Europe and undermining global peace and security,” he said. “In exchange, [President] Putin is providing North Korea with military technology and other support to circumvent international sanctions.”

“Over 600,000 Russian soldiers have been killed or wounded in [President] Putin’s war, and he is unable to sustain his assault to Ukraine without foreign support,” Rutte added.