Volodymyr Zelenskyy accused the US of "constantly changing" a minerals deal linked to military aid, as cracks in the already fractured relationship between Kyiv and Washington appeared to widen on Thursday.
Speaking after a meeting in Paris, the Ukrainian President also said America's stance against Russia, which launched a full-scale invasion of his country three years ago, should be stronger.
"The conditions are constantly changing," Mr Zelenskyy said, of negotiations with the US about the minerals deal.
"I thought that we have agreed that there will be a framework deal, and then the full deal.
"Now, as I understand, the Ukrainian and American teams are working on it, because the US is changing these conditions and offers to sign the full deal right away."
The agreement would see the US get a future 50 per cent stake in Ukraine's vast, state-owned mineral wealth as well as oil and gas reserves.
It would be considered payment for the military and financial aid received since Russia's full-scale invasion three years ago.
Mr Zelenskyy's comments come amid a tumultuous period for Ukrainian-US relations.
American President Donald Trump has criticised his Ukrainian counterpart on numerous occasions and an awkward Oval Office exchange last month meant the world saw how fraught their relationship had become.
Mr Zelenskyy had travelled to Washington to sign the minerals deal, although after the public argument, it didn't happen and Mr Trump retaliated by temporarily cutting off military aid and intelligence to Ukraine.
While speaking in Paris, Mr Zelenskyy said his country needed US support if it was going to be in a strong negotiating position as talks aimed at brokering a ceasefire with Russia continue.
He also said he didn't want his comments about the minerals deal to be interpreted as an indication he was against the agreement.
Mr Zelenskyy was in Paris as part of a meeting of the so-called "coalition of the willing", being led by France and the United Kingdom.
Nearly 30 countries were represented, as well as NATO and European Union chiefs.
The group aims to bolster military and financial aid to Ukraine as it fights Russia's invasion, and provide security after the war ends.
French President Emmanuel Macron and British Prime Minister Keir Starmer are attempting to garner support for a multi-national peacekeeping force to be deployed to Ukraine after a ceasefire is declared, but so far, public commitments from other countries with regard to sending soldiers are scarce.
Australian Prime Minister Anthony Albanese has previously said he'd consider putting boots on the ground in Ukraine after a ceasefire is declared.
The US has convened separate ceasefire talks with delegations from Russia and Ukraine, but progress has been slow.
Marie Dumoulin, the director of the Wider Europe programme at the European Council on Foreign Relations, told the ABC the coalition being formed was an important part of Ukraine's security.
"There are talks going on between the United States, Russia and Ukraine and Europeans are not involved," she said.
"The coalition that is building up is a way to signal that there are a number of countries willing to offer Ukraine options for its long-term security."
On Thursday, European nations at the Paris meeting agreed to supply two million rounds of ammunition to Ukraine.
Germany and France have this week also pledged to send €3 billion ($5.14 billion) and €2 billion ($3.42 billion) respectively in additional aid to Ukraine.
Speaking after the Paris meeting, Mr Starmer accused Russia of "filibustering" ceasefire negotiations.
It's understood Mr Trump and Mr Macron spoke via phone as part of Thursday's meeting, as the French leader tries to shore up support for sending troops to Ukraine.
"My wish is that the Americans are engaged at our side," Macron said after the meeting
"But we have to be prepared for a situation in which they maybe don't join in."
David Cadier, a research fellow at the Insitute of Strategic Studies — a defence and security think tank — said it was unsurprising not all countries at the Paris meeting had made public commitments to put troops in Ukraine.
"Where there might be disagreements and the divergences will rather be in relation to the individual contribution of each country," he told the ABC, adding that deploying soldiers was not the only way to show support.
"Contributions can vary, on land, on the sea and in the air," he said.
Russian Foreign Ministry spokeswoman Maria Zakharova on Thursday claimed France and the UK were planning a "military intervention in Ukraine" and warned it could lead to a direct flashpoint between Russia and NATO.
