Kyiv -- backed by NATO allies in eastern Europe -- has called for a commitment at the gathering in Lithuania in two weeks that it will join the Western military alliance when Russia's war ends.

Diplomats at NATO say the US has been reluctant to move beyond a 2008 pledge made in Bucharest that promised Ukraine would become a member, but did not set any timeline.

NATO's 31 countries are currently haggling over the exact wording on Ukraine's potential membership for a final summit communique.

US ambassador to NATO Julianne Smith said the final version could begin to answer how Ukraine will eventually become an alliance member.

"I think most of us feel confident that we are going to be able to come to an agreement that will reflect where we are and that the Ukrainians will believe and feel is something above and beyond restating Bucharest," she said.

Smith said the alliance was getting closer to finding a consensus on the language, but said she did not want to pre-empt the final phrasing.

She said that eventually allowing Ukraine to skip a Membership Action Plan, laying out benchmarks to be met for membership, was "one option that we're looking at".

Multiple Western diplomats say NATO heavyweights the United States, Britain, Germany and France are currently discussing bilateral commitments on weapons supplies with Ukraine as an interim assurance before membership.

Smith said Washington was having conversations with Kyiv on "reassuring them over the long term and helping them along the lines of what we've been doing for the last 15 months".

She gave no indication on whether any concrete announcements could be made ahead of the Vilnius summit.

"When you take all of this together, I think it will give the Ukrainians the feeling that we've heard them and that we understand that they were looking for something concrete," Smith said.

The US envoy said she expects Ukrainian President Volodymyr Zelensky to attend the gathering next month and that his presence send would send a "powerful" message to Russian leader Vladimir Putin.

"The signal coming out of Vilnius will be we continue to stand behind Ukraine, we stand with Ukraine, and having Zelensky there in person, I think, again, will be very powerful.

"I think it's important for the Ukrainians, and I think it's important for the NATO alliance."