Ukrainian President Volodymyr Zelensky is set to attend the two-day gathering in Lithuania to make the case that his country should join when Russia's war ends.

"I expect allied leaders will reaffirm that Ukraine will become a member of NATO and unite on how to bring Ukraine closer to its goal," Stoltenberg said.

NATO's 31 members are still haggling over the final wording for a communique on how to address Ukraine's membership.

Eastern European countries are backing Kyiv's calls to be granted a pathway into NATO for when the conflict ends.

But key powers the United States and Germany have been reluctant to go beyond a 2008 promise that said Ukraine would join one day, without setting a clear timeframe.

"The exact wording will be made public when we have agreed," Stoltenberg said.

NATO diplomats say allies could drop more stringent conditions for Ukraine when it does eventually get invited to join.

Stoltenberg said, as part of a package, NATO would tighten ties with Kyiv by launching a NATO-Ukraine Council that will sit for the first time in Vilnius with Zelensky participating.

He said NATO members have also committed 500 million euros ($545 million) to a multi-year programme aimed at bringing Ukraine closer to alliance standards.

To reassure Kyiv in the interim before it joins NATO, diplomats say heavyweights the United States, Britain, France and Germany are negotiating with Ukraine over longer-term commitments to supply arms.

These would be outside the NATO framework and fall far short of Kyiv's ultimate desire to be covered by the alliance's collective-defence umbrella.

Stoltenberg said any such assurances would "complement and underpin" the work NATO is doing to bring Ukraine closer.