"Our side is ready to sign. The Ukrainians decided last night to make some last-minute changes," US Treasury Secretary Scott Bessent told reporters at the White House.
"We're sure that they will reconsider that and we are ready, if they are."
A Ukrainian official was on the way to Washington DC for the signing.
But a source said the US was pushing Ukraine to sign two additional documents and that Ukrainian officials felt it was premature.
Bessent denied the US made any attempt to change the accord the two sides agreed to over the weekend.
The agreement, which would give the United States access to Ukraine's minerals deposits, is central to Ukrainian efforts to mend ties with US President Donald Trump and the White House as Trump tries to secure a peace settlement in Russia's war in Ukraine.
Two sources told Reuters the signing could happen later on Wednesday.
Ukraine's First Deputy Prime Minister Yulia Svyrydenko was flying to the United States to sign the deal, the country's prime minister said earlier.
A draft of the main minerals agreement seen by Reuters showed Ukraine had secured the removal of any requirement for it to pay back the US for past military assistance, something Ukraine had staunchly opposed.
The US has been Ukraine's single largest military donor since Russia's 2022 invasion with aid of more than 64 billion euros ($A114 billion), according to the Kiel Institute in Germany.
Trump repeated on Wednesday that the US should get something for its prior aid to Ukraine, thus the effort to secure a deal for Ukraine's plentiful deposits of rare earth minerals.
"I assume they're going to honour the deal. ... We haven't really seen the fruits of that deal yet. I suspect we will," Trump said after a cabinet meeting on Wednesday.
Ukrainian officials hope that signing the deal promoted by Trump will firm up US support for Ukraine in the more than three-year-old war.
The draft agreement gives the US preferential access to new Ukrainian natural resources deals but does not automatically hand the US a share of Ukraine's mineral wealth or any of its gas infrastructure, the draft showed.
"Indeed, it is a strategic agreement ... it is a truly equal, good international agreement on joint investments in the development and recovery of Ukraine," Prime Minister Denys Shmyhal said.
The minerals draft sets out the creation of a joint US-Ukrainian fund for reconstruction which will receive 50 per cent of profits and royalties accruing to the Ukrainian state from new natural resources permits in Ukraine.
The draft does not spell out how the joint fund's revenues will be spent, who benefits or who controls decisions about the spending.
Shmyhal said in televised comments that once the main agreement was signed, the two sides would agree two further technical and supplementary documents outlining issues such as how the funds are accumulated.
He said Ukraine would retain control of all its resources in the deal while the fund will invest in the development of Ukraine for 10 years.
Previous iterations of the deal during negotiations had said that it would include Ukrainian natural gas infrastructure, which is one of Ukraine's most valuable assets, sources with knowledge of the matter said earlier this month.
Gas infrastructure is not cover by the draft seen by Reuters on Wednesday.
The Ukrainian cabinet was expected to approve the agreement's text on Wednesday, after which it will be signed by an authorised government representative.
The deal will then require ratification in the Ukrainian parliament before it can take effect.
with AP
