NATO members are supplying Ukraine with military aid, seen as essential to defend itself in the conflict with Russia, and Kyiv is pushing to be allowed to join the transatlantic alliance.

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

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 in the conflict with Russia.

Russia says NATO's deliveries of military aid to Kyiv are proof that the alliance and the United States are waging a war by proxy in Ukraine.

The Kremlin said on Thursday that preventing Ukraine joining the Western military alliance remains one of the main aims of its invasion.

"Otherwise, it will pose a serious danger to our country, to its security," the Kremlin said.

Russian President Vladimir Putin has claimed he had no choice but to send troops into Ukraine to stop it moving into the NATO fold.

Since Moscow's invasion, Ukraine has been seeking to be allowed into the alliance but Western diplomats say that still remains a distant prospect.

Earlier this month, Stoltenberg invited Ukrainian President Volodymyr Zelensky to attend a NATO summit, scheduled for July. He said Putin had "wanted to slam NATO's door shut" but had "failed".

Zelensky has called for Ukraine to be allowed "accelerated NATO membership", while Stoltenberg has implied that Kyiv must first win the war against Moscow.