North Korean troops have been deployed to Russia and are operating in the Kursk border region where Ukrainian troops have a foothold, Nato has said for the first time.

The alliance’s Secretary General Mark Rutte said he could confirm the deployment after weeks of speculation, following a meeting with South Korean intelligence and defence officials on Monday.

The newly installed Nato chief said the deployment represented a “significant escalation” and a “dangerous expansion” of Russia’s war in Ukraine.

Last week, President Vladimir Putin refused to deny that North Korean troops had arrived in Russia, following reports that Pyongyang was preparing to send thousands of troops to aid its ally.

South Korea’s spy agency said earlier this month that at least 1,500 North Korean troops had arrived in Russia, prompting Seoul to issue Moscow with a stern diplomatic rebuke.

But Rutte’s intervention on Monday marked the first time Nato had formally acknowledged that Pyongyang’s forces were operating in Russia. He added that North Korea has already sent millions of rounds of ammunition and ballistic missiles to Moscow for use in Ukraine.

In return, President Putin has agreed to send military technology and other support to help North Korea evade international sanctions, Rutte said. The partnership, he added, was “undermining global peace and security”.

His warning that North Korean troops are operating in Kursk will cause concern in Western capitals. Clashes have continued between Moscow and Kyiv’s forces almost two months after Ukrainian troops first entered the region in a shock operation.