The military alliance has bolstered its eastern defences since Russia invaded Ukraine in February 2022, with Finland and Sweden also overhauling decades of security policy to join the alliance.

The summit brings together the Bucharest Nine -- the alliance's members across eastern and central Europe -- with its Nordic members, Zelensky and NATO Secretary General Mark Rutte.

Zelensky's spokesman said he would hold "bilateral meetings" on the sidelines of the summit in the Lithuanian capital.

It comes ahead of a full NATO summit later in June in The Hague to which Zelensky has demanded he be invited to.

"If Ukraine is not present at the NATO summit, it will be a victory for Putin, but not over Ukraine, but over NATO," he said last week.

Zelensky wants NATO to offer security guarantees to Ukraine in the event of a ceasefire or peace deal with Russia -- something Moscow has called "unacceptable."

NATO's eastern members have been some of the strongest backers of Ukraine since Russia invaded and have repeatedly warned about the prospect of Moscow stepping up its aggression.

Baltic states Lithuania, Estonia and Latvia are former Soviet republics, now EU members, that fear they are in Moscow's crosshairs.

US President Donald Trump has heaped pressure on NATO's European members to increase their defence spending, sparking fears about the US commitment to protect the continent.