Ukraine's incursion into Russia's Kursk region is legitimate and covered by Kyiv's right to self-defence, NATO Secretary-General Jens Stoltenberg told German weekly Welt am Sonntag in his first reaction to the advance into Russian territory.

"Ukraine has a right to defend itself. And according to international law, this right does not stop at the border," Stoltenberg told the paper, adding that NATO had not been informed about Ukraine's plans beforehand and did not play a role in them.

The NATO chief said Ukraine was running a risk with the advance onto Russian territory but that it was up to Kyiv how to conduct its military campaign.

"(Ukrainian) President (Volodymyr) Zelenskiy has made clear that the operation aims to create a buffer zone to prevent further Russian attacks from across the border," he said.

"Like all military operations, this comes with risks. But it is Ukraine's decision how to defend itself."

The NATO chief said Ukraine was running a risk with the advance onto Russian territory but that it was up to Kyiv how to conduct its military campaign.

"(Ukrainian) President (Volodymyr) Zelenskiy has made clear that the operation aims to create a buffer zone to prevent further Russian attacks from across the border," he said.

"Like all military operations, this comes with risks. But it is Ukraine's decision how to defend itself."

Russia has called the Kursk operation a "major provocation" and said it would retaliate.