The initiative -- to be unveiled at a summit in Washington next week -- was pitched by NATO chief Jens Stoltenberg as a way to firm up Western aid for Kyiv in its war with Russia for the coming years.

The plan was to get countries to vow to keep on giving support at the rate they've been giving since Moscow invaded, and to split contributions more equitably.

But diplomats said leading power the United States insisted that the pledge -- which is not legally binding -- should be reviewed next year.

Washington's argument was that it was not legally possible for the government to commit any future administrations to spending, diplomats said.

Diplomats also said that a proposal to lay out a clear way to split future aid according to the size of each country's gross domestic product was dropped due to opposition led by Turkey.

A diplomat said there was a reference to "fair burden sharing" in the final text and the aim to contribute more proportionally, but no mention of using GDP as a scale.

The pledge is one part of a package for Ukraine that NATO is hammering out for the summit in Washington.

Kyiv is also set to get stronger wording in a final declaration on its push to join, but no concrete invite to become a member.

The alliance will also look to insulate weapons deliveries against any possible return to the US presidency by Donald Trump by taking control of coordination of supplies from Washington.