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 intelligence reports, following a meeting with South Korean security and defense 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. "This is our sovereign decision," Putin said, sidestepping the question during a news conference. "Whether we use it or not, where, how, or whether we engage in exercises, training, or transfer some experience. It’s our business." It is unclear exactly how many North Korean soldiers have been sent. South Korea"s spy agency said earlier this month that at least 1,500 North Korean troops had already 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 had already sent ballistic missiles and millions of rounds of ammunition 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. Fighting has continued between Moscow and Kyiv"s forces more than two months after Ukrainian troops first entered Russia"s western region in a shock operation. Russia is said to have redeployed thousands of troops into the region, helping to stall Ukraine"s advance. The operation has seen Kyiv"s forces claim about 250sq km of territory, but it appears to have failed in its primary objective of diverting Moscow"s momentum in the east of Ukraine. The arrival of North Korean forces in Kursk could heap further pressure on Kyiv"s embattled troops. A senior Ukrainian official told the New York Times that about 5,000 elite North Korean troops were set to have joined the Russian detachment in the border region by Monday. President Volodymyr Zelensky also said on Friday that his government had information that those troops could be on the battlefield within days. Western leaders have warned for weeks that such a move would risk an intensification of the conflict. Last week, Belarusian leader Alexander Lukashenko - a loyal ally to President Putin - appeared to echo that assessment. He told the BBC that such a move would mark "a step towards the escalation of the conflict". North Korea and Russia have grown increasingly close since Moscow found itself largely isolated after its February 2022 invasion of Ukraine. Earlier this year, North Korea"s Supreme Leader Kim Jong Un struck an agreement with President Putin pledging to help each other in the event of "aggression" against either country. The US has repeatedly accused Pyongyang of sending vast amounts of military hardware to Russia, including ballistic missiles and launchers. But some experts have questioned the degree to which Pyongyang"s troops will be able to aid the Russian war effort. Apart from the language barrier, the North Korean army has no recent combat experiences, they said. Footage obtained by Ukrainian defence intelligence officials has also showed Russian troops expressing doubts as to how the North Korean troops will be commanded and supplied. Moscow"s full-scale invasion has now raged for more than two-and-a-half years, with Rutte claiming that more than 600,000 Russian troops have now been killed or wounded in the war. He said the Kremlin was "unable to sustain his assault on Ukraine without foreign support". Separately, President Zelensky said on Monday that about 650,000 Russian soldiers had been killed or wounded. "They [Russians] are not collecting the bodies... their people are rotting on the ground," he said in an interview with The Times of India. Official casualty updates from either side are rare. But according to an analysis by BBC Russian, more than 70,000 Russian troops have been confirmed killed in combat. In February, Zelensky said about 31,000 Ukrainian soldiers had been killed since Russia"s full-scale invasion. Estimates by several Western media outlets suggest that number is much higher. — BBC