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.

"I'm going to be announcing very shortly a major speech I'm going to make on this issue, and why it's critically important for the United States and our allies that we keep our commitment," Biden said.

Biden declined to say when he would make the speech.

Russia has said that the questions over the future of US aid reflect growing fatigue in the West over its support for Ukraine.

But the White House insisted that there were no cracks in the alliance when Biden spoke to the leaders of key European allies and others on Tuesday.

"None of them brought up that they were concerned," Press Secretary Karine Jean-Pierre told reporters. 

"They have their own domestic political issues that they have to deal with as well."

German Chancellor Olaf Scholz, who was among those who spoke with Biden, said on Wednesday he was "convinced" of continued US support for Ukraine. 

The United States is by far the biggest supporter of Kyiv, committing more than $43 billion in military assistance to Kyiv so far, while Congress has approved a total of $113 billion in aid including humanitarian help.

Without new aid being approved, the funding could run out in a "couple of months," National Security Council spokesman John Kirby said.

In a statement Wednesday, the US military said it had given Ukraine's armed forces more than 1 million rounds of seized Iranian ammunition.

But the White House's Jean-Pierre said she "wouldn't connect" this with concerns over the future of US aid.

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