The Kyiv Independent newspaper published images of Stoltenberg in central Kyiv, in front of a memorial for fallen soldiers.

The photographs showed Stoltenberg with his head down in front of the memorial.

Many Western officials travelling to Kyiv do not announce their trips in advance for security reasons.

"The NATO Secretary General is in Ukraine. We will release more information as soon as possible," a NATO official said.

Stoltenberg's visit comes 14 months into Russia's invasion and ahead of an expected Ukrainian counter-offensive.

NATO has pledged to stand by Ukraine as it fights Russia.

Earlier this month, Stoltenberg invited Ukrainian President Volodymyr Zelensky to a NATO summit in July.

He also said Russian President Vladimir Putin had "wanted to slam NATO's door shut" but "failed."