The United States and Ukraine have hailed "productive" talks on Washington's plan to halt Ukraine war, but both sides also cautioned that the high-stakes negotiations were far from over.

US Secretary of State Marco Rubio said on Sunday that more work was required, and a source in Kiev's delegation characterised the discussions as "not easy."

Washington has put forward a plan to end the nearly four-year conflict and is seeking to finalise it with Moscow and Kiev's approval.

The negotiations, which follow talks in Geneva, could set the stage for an upcoming visit to Moscow by President Donald Trump's envoy Steve Witkoff, who is expected to discuss Ukraine with Russian President Vladimir Putin.

"We had another very productive session, building off Geneva, building off the events of this week," Rubio told reporters.

"But there's more work to be done. This is delicate. It's complicated," he added.

"There are a lot of moving parts, and obviously there's another party involved here that will have to be a part of the equation, and that will continue later this week when Mr Witkoff travels to Moscow."

Later, after Rubio's remarks, President Donald Trump said there was a good chance of ending the war after the talks, but said the corruption scandal doesn't help.

A US official said that Witkoff will travel to Moscow on Monday for expected talks with Russian President Vladimir Putin.