US President Donald Trump is "extremely frustrated" with Russia and Ukraine, his spokeswoman said on Thursday, as Kyiv said Washington was still pushing it to make major territorial concessions as part of its plan to end the nearly four-year conflict.

"The president is extremely frustrated with both sides of this war," Karoline Leavitt told reporters. "He doesn't want any more talk. He wants action. He wants this war to come to an end."

Earlier, Ukrainian President Volodymyr Zelensky made remarks that appeared to show little had changed in Washington's core position on how the conflict should end since it sent a 28-point plan to Kyiv and Moscow last month that heavily favoured Russia.

Zelensky said that Washington was still pushing it to cede land to Russia as part of an agreement to end the war that started with Moscow's February 2022 assault.

Washington wants only Ukraine, not Russia, to withdraw its troops from parts of the eastern Donetsk region, where a demilitarised "free economic zone" would be installed as a buffer between the two armies, Zelensky told reporters.

Under the latest US plan, Moscow would also stay where it is in the south of the country, but pull some of its troops out of Ukrainian regions that Russian President Vladimir Putin has not claimed to have annexed in the north.

Ukraine has been revising the original US proposal and this week sent a 20-point counter-proposal to Washington, the full details of which have not been published.

"We have two key points of disagreement: the territories of Donetsk and everything related to them, and the Zaporizhzhia Nuclear Power Plant (in the south). These are the two topics we continue to discuss," Zelensky told reporters at a briefing.

"They see Ukrainian forces leaving the territory of Donetsk region, and the supposed compromise is that Russian forces do not enter this territory... which they already call a 'free economic zone'," Zelensky said about the US plan.

Zelensky has long said he has no "constitutional" or "moral" right to cede Ukrainian land and said on Thursday that Ukrainians should have the final word.

"Whether through elections or a referendum, there must be a position from the people of Ukraine," he said.

Zelensky also pushed back against the idea of a unilateral Ukrainian withdrawal in the Donetsk region.

"Why doesn't the other side of the war pull back the same distance in the other direction?" he said, adding there were "a great many questions" still unresolved.

Under the US plan, Russia would relinquish territory it has captured in the Kharkiv, Sumy and Dnipropetrovsk regions – three areas over which Moscow has not made a formal territorial claim.

Much of eastern and southern Ukraine has been decimated by fighting.

Tens of thousands have been killed and millions forced to flee their homes.

Russia, which has the numerical advantage in manpower and weapons, has been grinding forward on the battlefield.

It claimed on Thursday to have captured the town of Siversk in the Donetsk region, where its army is advancing at its fastest pace in a year, according to independent analysis.

Ukraine's army's eastern command denied the Siversk claim. (AFP)