US President Donald Trump has said it was up to Europe to offer security guarantees to Ukraine, declining to make US promises as part of a deal for Ukraine's natural resources.

"I'm not going to make security guarantees beyond very much," Trump said at a cabinet meeting on Wednesday.

"We're going to have Europe do that because... Europe is their next-door neighbour, but we're going to make sure everything goes well."

Meanwhile, Ukrainian President Volodymyr Zelenskyy insisted that security guarantees were vital ahead of talks in Washington on Friday.

"Guarantees of peace and security are the key to preventing Russia from destroying the lives of other nations," Zelenskyy said in his evening video address.

"For me and for all of us in the world, it's important that US support is not stopped. Strength is needed on the path to peace."

Fraught discussions

Discussions were fraught on the minerals deal, which would grant the United States preferential access to Ukrainian natural resources in exchange for US security support.

Officials late on Tuesday said they had come to an agreement following protracted negotiations, but Zelenskyy told reporters in Kiev that more difficult work lay ahead.

"This is a start, this is a framework agreement," he told journalists.

Further discussions between US and Ukrainian officials would determine the nature of security guarantees for Ukraine and the exact sums of money at stake in the accords.