Ukrainian President Volodymyr Zelensky is "not ready" to sign a minerals deal with the United States, a source told AFP on Saturday, despite growing pressure from US officials and President Donald Trump.

The Republican has upended US foreign policy since taking office last month, making diplomatic overtures towards the Kremlin while demanding Ukraine give up its rare minerals to compensate for the wartime aid it received under Joe Biden.

Trump's special envoy Keith Kellogg said Saturday that Zelensky understood signing a deal with the US was "critical", but a Ukrainian source told AFP that Kyiv needed assurances first.

"In the form in which the draft is now, the president is not ready to accept, we are still trying to make changes and add constructiveness," the source close to the matter said.

Ukraine wants any agreement signed with the US to include security guarantees as it battles Russia's nearly three-year invasion.

The negotiations between the two countries come amid a deepening war of words between Trump and Zelensky that has raised alarm in Kyiv and Europe.

On Wednesday, Trump branded his Ukrainian counterpart a "dictator" and called for him to "move fast" to end the war, a day after Russian and US officials held talks in Saudi Arabia without Kyiv.

On Friday, the US proposed a United Nations resolution on the Ukraine conflict that omitted any mention of Kyiv's territory occupied by Russia, diplomatic sources told AFP.

Trump has asked for "$500 billion worth" of rare earth minerals to make up for aid given to Kyiv -- a price tag Ukraine has balked at and which does not correspond with published US aid figures.

"There are no American obligations in the agreement regarding guarantees or investments, everything about them is very vague, and they want to extract $500 billion from us," the Ukrainian source told AFP of the proposed deal.

"What kind of partnership is this? ... And why do we have to give $500 billion, there is no answer," the source said, adding that Ukraine had proposed amendments to the draft.

The United States has given Ukraine more than $60 billion in military aid since Russia's invasion, according to official figures -- the largest such contribution among Kyiv's allies but substantially lower than Trump's $500 billion figure.

The Kiel Institute, a German economic research body, said that from 2022 until the end of 2024, the United States gave 114.2 billion euros ($119.8 billion) in financial, humanitarian and military aid in total.

A senior Ukrainian official told AFP Friday that despite the tensions, talks on a possible agreement were "ongoing", with Kellogg praising Zelensky as "courageous" after visiting Kyiv earlier this week.

The row comes at a critical moment in the conflict, with Ukraine marking the third anniversary of Russia's invasion on Monday and Kyiv's forces slowly ceding ground on the frontline.

Moscow's defence ministry earlier on Saturday claimed the capture of Novolyubivka in the eastern Lugansk region, which is now largely under Russian control.

In a call with Zelensky on Saturday, British Prime Minister Keir Starmer pledged the "UK's ironclad support for Ukraine".

Zelensky, in response, praised the United Kingdom for showing "leadership" on the war with Russia.

In London, thousands of people marched in support of Ukraine on Saturday, and polls in the UK suggest strong support for Kyiv. (AFP)

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