President Joe Biden admitted Wednesday he was worried that political turmoil in Washington could threaten US aid to Ukraine, urging Republicans to stop their infighting and back "critically important" assistance for Kyiv.

Biden added that he would soon be giving a major speech on the need to support Ukraine's fight against the Russian invasion after the chaos in Washington alarmed US allies.

"It does worry me," Biden told reporters when asked whether the ousting of Republican House speaker Kevin McCarthy by hardliners in his own party could derail more funds for Ukraine's war effort.

"But I know there are a majority of members of the House and Senate of both parties who have said that they support funding Ukraine."

A last-gasp deal in Congress to avoid a US government shutdown at the weekend contained no fresh funding for Ukraine, and hopes for a quick solution have been further complicated by McCarthy's exit on Tuesday.

The contenders to replace him hold a range of views but among them is hard-right Republican Jim Jordan, who has been notably skeptical on funding Ukraine.

The timing is critical, with the White House warning that aid could run out within months just as Ukraine tries to push forward its slow-moving offensive against Russia before winter sets in.

Biden indicated there was "another means by which we may be able to find funding" without congressional approval, but would not give further details.

The president will get a briefing on Ukraine from his national security team on Thursday, the first to feature the new top US military officer, General Charles "CQ" Brown, the White House added.

'Keep our commitment'

The president's comments reflected a change of tone, as Biden had told allies in a call on Tuesday that he was "confident" of getting fresh aid passed, according to the White House.

The US president said he would now make the case for the importance of helping Ukraine as it battles the full-scale invasion launched by Russia in February 2022.