"All allies agree that NATO's door remains open, that Ukraine will become a member of the alliance and that Russia does not have a veto," he said.

The NATO chief was in the Slovakia capital Bratislava for talks with the presidents of the alliance's eastern flank, the so-called Bucharest Nine group.

Russia's invasion of Ukraine in February last year has galvanised NATO, set up almost 75 years ago to face off against the Soviet Union.

The Bratislava talks came ahead of next month's NATO summit in Vilnius, where Stoltenberg said the alliance will take "very significant" decisions.

"We will strengthen our support for Ukraine with a multi-year package of assistance to help them transition from Soviet-era to NATO standards, and bring Ukraine closer to NATO," he said.

"We must also agree a stronger commitment to increase defence investments, with two percent of GDP as a minimum," he added.

Stoltenberg also said he was "working hard to ensure that we welcome Sweden soon as a full member of this alliance".

Sweden and Finland dropped decades of military non-alignment and applied to join the alliance after Russia's invasion of Ukraine.

Finland formally joined NATO in April.