NATO chief Mark Rutte confirmed the presence of North Korean troops in Russia's Kursk region, as Ukraine called on allies to send more weapons and allow deep strikes into Russia.

"The deepening military cooperation between Russia and North Korea is a threat to both Indo-Pacific and Euro-Atlantic security," Rutte told reporters after NATO officials and diplomats were briefed by a South Korean delegation.

Ukrainian military intelligence said on Thursday that the first North Korean units had already been recorded in the Kursk border region, where Ukrainian troops have been operating since staging a major incursion in August.

Rutte said the North Korean deployment represented "a significant escalation" of Pyongyang's involvement in "Russia’s illegal war" in Ukraine, a breach of U.N. Security Council resolutions and a "dangerous expansion" of the conflict.

Ukrainian Foreign Minister Andrii Sybiha said that Kyiv had been warning about the deployment for weeks, yet there was no strong response from allies.

"Now NATO Secretary General confirmed this. The bottom line: listen to Ukraine. The solution: lift restrictions on our long-range strikes against Russia now," he said on X.

The Kremlin had initially dismissed reports about a North Korean deployment as "fake news". But Putin on Thursday did not deny that North Korean troops were currently in Russia and said that it was Moscow's business how to implement a partnership treaty with Pyongyang.

A North Korean foreign ministry official did not confirm media reports about a troop deployment to Russia but said if Pyongyang had taken such action, he believed it would be in line with international norms.

The deployment of North Korean troops was a sign of "growing desperation" on the part of Putin, Rutte said.

"Over 600,000 Russian soldiers have been killed or wounded in Putin’s war and he is unable to sustain his assault on Ukraine without foreign support," Rutte said.

The Ukrainian president's chief of staff Andriy Yermak said sanctions alone would not be a sufficient response to North Korean involvement.

He added that Kyiv needs "weapons and a clear plan to prevent North Korea's expanded involvement."

"The enemy understands strength. Our allies have this strength," Yermak said on X.