"As for the summit this summer, I do not expect the alliance to issue an invitation," US envoy Julianne Smith told journalists.

NATO countries last year reiterated a long-standing position that Kyiv will one day become a member of the US-led military alliance, but did not set a clear timeframe.

Smith said she believes this year's summit in Washington will "signal that the alliance continues to move closer to Ukraine and that we are taking concrete steps to serve as a bridge between where we are now and full-fledged membership".

Ukraine has strong backing for its NATO push from countries on the eastern flank of the alliance who feel themselves more directly threatened by Moscow.

But key powers such as the United States and Germany are wary of moving the war-torn country towards full membership for fear it will drag NATO into a direct conflict with Russia.

The United States is working "to help our friends in Ukraine with the necessary reforms inside their own country to move closer to Euro-Atlantic integration," Smith said.

"We continue to focus first and foremost on supporting them in the current fight and ensuring that they can prevail on the battlefield."

Ukraine's international backers will hold talks on Wednesday aimed at rallying support for Ukraine as doubts swirl over the future of US aid.

The US Senate on Tuesday approved long-delayed funding for Ukraine's war effort, but the package looks set to be rejected by the Republican-led lower house.