NATO chief Jens Stoltenberg visited Kyiv on Thursday for the first time since Russia's full-scale invasion, showing the military alliance's support for Ukraine as it prepares to launch a counteroffensive.

Stoltenberg laid a wreathe to honour Ukrainian soldiers who have been killed fighting in the east of the country, and reviewed captured Russian armoured vehicles on the capital's St Michael's Square.

Ukrainian leaders and NATO officials did not immediately make any announcements about the trip. Wartime visits by foreign officials are often shrouded in secrecy but top leaders visiting Kyiv often hold talks with President Volodymyr Zelenskiy.

Stoltenberg began his unannounced trip at a vital juncture in Russia's almost 14-month-old invasion which has killed thousands, uprooted millions, destroyed cities and devastated the Ukrainian economy.

After weathering a Russian winter and spring offensive that has made only small advances in the east, Ukraine now hopes to retake land in its south and east in a counteroffensive in the coming weeks or months.

After paying his respects to Ukrainian soldiers, the NATO secretary-general got into a car and drove off after the event, a Reuters photographer said.

The North Atlantic Treaty Organization has supported Ukraine throughout the war, with member states sending weapons but not fighting troops. Kyiv has repeatedly called for more weapons from its allies.

Ukraine sees its future in the alliance and last September announced a bid for fast-track membership after the Kremlin said it had have annexed four Ukrainian regions that its troops have partially occupied.

Moscow regards NATO as a hostile military bloc bent on encroaching on what it sees as its sphere of influence. Ukraine gained independence from the Russia-led Soviet Union in 1991.

Russia did not immediately comment on Stoltenberg's visit.