"We are all moving with haste to facilitate this and get this done, and, you know, I think things are actually moving very quickly," US ambassador to NATO Matthew Whitaker told journalists.

"But I can't verify a date that this will all be completed."

Trump on Monday announced a deal with NATO chief Mark Rutte for European NATO states to buy US weaponry -- particularly advanced Patriot systems -- and give it to Kyiv.

The move marked a pivot for the US leader as his patience has worn thin with Russia's President Vladimir Putin for frustrating efforts to halt the war in Ukraine.

European countries -- including Germany, Norway, the Netherlands, Denmark and Sweden -- have expressed willingness to buy the weaponry for Kyiv.

But questions have been raised over where the systems will come from and how soon they can get to Ukraine as the war-torn country faces unprecedented Russian bombardments.

Whitaker said that there was an "ongoing conversation" about whether the United States would sell Patriots already available in its own military stockpiles.

"We're never going to put the United States at a strategic disadvantage, and we're going to make sure that we have everything we need," he said.

"At the same time, I think we all acknowledge the desperate requirements that Ukraine currently, immediately needs on the battlefield and to protect their cities."

Other options mooted include European countries sending their systems to Kyiv now, and being able to purchase replacements as a priority from the United States.

The deal comes as US allies nervously monitor a US review of its force deployments around the world.

Washington has warned it could look to shift forces and weaponry away from Europe to focus more on China.

Whitaker said the United States was in "daily conversations" with allies about the process ahead of possible announcements in coming months.

"We've agreed to no surprises and no gaps in the strategic framework of Europe," he said.

"I don't think there's a lot of anxiety. I think there's just a lot of desire to know our plan is so that there can be other planning."