US Secretary of State Marco Rubio says talks with Ukraine over a deal to end the Russia-Ukraine war were productive, but there's "more work to be done".

The talks, held in Florida, were attended by a Ukrainian delegation led by national security council secretary Rustem Umerov, Ukraine's new chief negotiator.

Also at the meeting were President Donald Trump's envoy Steve Witkoff and Trump's son-in-law, Jared Kushner. Witkoff is due to visit Moscow for talks next week.

Umerov was brought in to replace the Ukrainian president's top negotiator, Andriy Yermak, after he quit following an anti-corruption raid on his home.

Sunday's meeting is the latest step in two weeks of intense diplomatic activity.

It was sparked by the leaking of a 28-point US peace plan, which shocked Ukraine and its European allies for appearing to favour Russia, which invaded Ukraine nearly four years ago.

"It's not just about the terms that end fighting," Rubio said at the meeting in Hallandale Beach, north of Miami.

"It's also about the terms that set up Ukraine for long-term prosperity... I think we built on that today, but there's more work to be done."

He also told the Ukrainian delegation that the aim of peace talks with Russia is to leave Ukraine "sovereign, independent and prosperous".

As Sunday's talks began, Umerov said: "We are discussing about the future of Ukraine, about the security of Ukraine, about no repetition of aggression of Ukraine, about prosperity of Ukraine, about how to rebuild Ukraine".

"US is hearing us," he said, speaking in English. "US is supporting us. US is working beside us."

He later described the talks as "productive and successful" while Rubio said they had been "very productive and useful".

Speaking to reporters on Air Force One on Sunday, Trump said the talks were "going along well", and there was a good chance of a deal being made to end the conflict.

Trump said earlier he would send Witkoff and possibly also Kushner to Moscow this week to meet President Vladimir Putin to discuss the peace plan, which has been much revised.

In a post on social media, Ukraine's President Volodymyr Zelensky said: "It is important that the talks have a constructive dynamic and that all issues were discussed openly and with a clear focus on ensuring Ukraine's sovereignty and national interests."

He added he was grateful to the US and Trump for their efforts to try and end the war.

One crucial unresolved issue is the fate of Ukrainian territory that Russia has either annexed or controls.

The talks in Florida were "not easy", a source who is said to be close to the Ukrainian delegation told AFP news agency.

"But everyone is trying to be constructive and find a solution," the source added.

Zelensky is due to visit Paris on Monday for talks with French President Emmanuel Macron, a strong ally.

Tens of thousands of soldiers have been killed or injured, as well as thousands of civilians, and at least seven million people have become refugees since Russia invaded on 24 February 2022.

The conflict between the two ex-Soviet states stretches back to 2014 when Ukraine's pro-Russian president was overthrown, and Russia responded by annexing Crimea and supporting armed uprisings in eastern Ukraine.