Speaking in Brussels after a briefing with South Korean intelligence officials, Rutte said he could confirm that North Korean military units had been sent to Russia's western Kursk region.

"The deepening military cooperation between Russia and North Korea is a threat to both the Indo-Pacific and Euro-Atlantic security," Rutte told a press conference.

"The deployment (of) North Korean troops to Kursk is also a sign of Putin's growing desperation."

Ukrainian troops have been conducting a ground offensive in Kursk since August and control several hundred square kilometres of Russian territory.

Rutte said more than 600,000 Russian soldiers have been killed or wounded since the conflict started, adding the Kremlin was unable to sustain the invasion without foreign support.

Growing military ties between Moscow and Pyongyang are a major concern for the United States and the European Union, as Putin presses ahead with his efforts to build an anti-Western alliance.

Last week, Ukrainian President Volodymyr Zelensky warned that Russia was planning to send North Korean troops into battle within days. He has claimed North Korea is training 10,000 soldiers to support Russia.

"This is an escalation. Sanctions alone are not enough. We need weapons and a clear plan to prevent North Korea's expanded involvement in the war in Europe," Zelensky's chief of staff Andriy Yermak said on social media Monday, referring to confirmation of the deployment.

"Today, Russia brings in North Korea; next, it could broaden their engagement, and then other autocratic regimes may see that they can get away with this and come to fight against NATO," he added.

"The enemy understands strength. Our allies have this strength."

South Korea, NATO and the United States have also said thousands of North Korean troops are training in Russia.

Rutte, a former Dutch prime minister who took over the reins of NATO this month, called on Moscow and Pyongyang to "cease these actions immediately".

NATO's Indo-Pacific partners, including Australia, Japan and New Zealand, have also been briefed on the situation, he said.