Ukraine said on Wednesday it had reached a "preliminary" deal to hand revenue from some of its mineral resources to the United States, before an expected trip to Washington by President Volodymyr Zelenskiy on Friday.

The deal is central to Ukrainian attempts to ensure strong support from U.S. President Donald Trump as he seeks a quick end to Russia's war in Ukraine, with U.S.-Russian talks that have so far excluded Kyiv set to continue on Thursday.

"The main thing for me is we are not debtors. There is no $500 billion debt in the agreement, nor $350, nor $100 billion because that would be unfair," Zelenskiy told a news briefing of the agreement.

Trump has cast the deal as a repayment for billions of dollars in aid to Kyiv during the war. Zelenskiy has been asking for security guarantees in exchange for the mineral rights, but it is unclear whether those demands have been successful.

"This agreement is part of our larger agreements with the United States. This agreement could be part of future security guarantees... an agreement is an agreement, but we need to understand the broader vision," Zelenskiy said.

"This deal could be a great success or it could pass quietly. And the big success depends on our conversation with President Trump."

Ukrainian Prime Minister Denis Shmyhal said Washington would commit to supporting Kyiv's efforts to obtain security guarantees under the finalised deal, but the Americans offered no security pledges of their own.

While discussions have been taking place on the minerals deal, Washington and Moscow have begun bilateral talks.

Russian Foreign Minister Sergei Lavrov said Thursday's talks in Istanbul would be on resolving bilateral disputes that are part of a wider dialogue the sides see as crucial to ending the Ukraine war.