US President Joe Biden will use his remaining months in office to strengthen Ukraine, a senior aide said on Saturday, as Kyiv fights off Russia's offensive for a third year.

Biden withdrew from the US elections, which are potentially perilous for Ukraine as they could see Ukraine-sceptic Donald Trump back in the White House.

The prospect has raised fears in Kyiv, which relies mainly on the US for support and is preparing for Biden's handover in January.

Biden is "determined to use the four months to put Ukraine in the best possible position to prevail," National Security Advisor Jake Sullivan said.

"President Zelensky has said that ultimately this war has to end through negotiations, and we need them to be strong in those negotiations," Sullivan said, adding Ukraine would decide when to enter negotiations with Russia.

Sullivan was speaking via video-link at a conference organised by the Victor Pinchuk Foundation.

The US President will meet with his Ukrainian counterpart Volodymyr Zelensky at the next UN General Assembly in late September, he said.

The two leaders will discuss how to best strengthen support to Ukraine, which is battling Russian advances in the east of the country.

"Ukraine has taken bold and assertive steps as well in the war. But that area around Pokrovsk is of unique concern," Sullivan said, referring to a logistics hub that Moscow's advancing forces are within 10 kilometres of.

Despite a surprise cross-border attack launched by Kyiv on August 6 in Russia's Kursk, Moscow continues to steadily gain ground in the eastern Donetsk region.

Sullivan said Washington paid particular attention to the situation in the east, and on helping defend Ukraine from "brazen" attacks on civilian energy infrastructure, potentially threatening nuclear power plants.

To fend off the renewed assaults in the east, Ukraine has called for prompt delivery of Western military aid.

Kyiv has often criticised delays that have held up the transfer of weapons from the West.

"It's not a matter of political will. It's a matter of busting through these difficult and complicated logistics," Sullivan said.

Sullivan nevertheless acknowledged that "given what Ukraine is up against, we've got to do more, and we've got to do better." (AFP)