Negotiations on a new United Nations resolution on disarming Iraq could continue into next week, according to UK and US officials.
The US is prepared for war with or without UN backing
"The diplomatic process is under way. And it may conclude tomorrow. It may continue into next week," White House spokesman Ari Fleischer said on Thursday.
The statement appears to mark a shift in US position following intense efforts from the UK to draft an Iraq resolution that can win broad support in the Security Council.
UK Prime Minister Tony Blair said on Thursday that the resolution appeared now "less likely than at any time", and war was getting closer, but his Foreign Office said talks were continuing.
But in Washington, Secretary of State Colin Powell told Congress that even though negotiations were continuing, the US might not even go for a vote at the Security Council.
"We are still talking to the members of the Council with respect to coalescing around a position that wouldn't draw a veto, but the options remain: go for a vote and see what members say, or not go for a vote," Mr Powell said.
The BBC's Jon Leyne in Washington says the UK attempt to put together a compromise has thrown the diplomatic game into confusion.
London has spoken of French "intransigence", after Paris rejected the latest British proposal which set out key tests for Iraqi disarmament.
Other Security Council members have given the plan a lukewarm reception.
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UK Foreign Secretary Jack Straw suggested that London would be flexible on at least one condition - that Saddam Hussein declare on television that he was abandoning forbidden weapons programmes.
White House officials are reported to be gearing up for a presidential address - possibly as early as this weekend - preparing the nation for war.
The speech is said to be likely to include an ultimatum for Saddam Hussein, giving him a date after which military action could be launched.
This would allow weapons inspectors and humanitarian workers to leave Iraq before hostilities start. The UN has already evacuated all of its observers from the Iraqi side of the demilitarised zone with Kuwait.
And the US has deployed B-2 stealth bombers for the first time, the French news agency AFP quoted an Air Force spokeswoman as saying.
The bombers have been used in combat before, but only flying sorties from their base in the United States. The spokeswoman did not say where the planes would be based.
Baghdad has dismissed the British key tests, saying they amount to a "dressing-up of a rejected proposal".
But Iraq is expected to hand to the UN a new batch of documents relating to its VX nerve agent programme before the end of the week.
Mr Blair held an emergency meeting with the leader of the UK opposition, Iain Duncan Smith, on Thursday.
Minutes later, Mr Straw attacked France's "extraordinary" rejection of Britain's latest proposals, saying it had been made "without even proper consideration".
French Foreign Minister Dominique de Villepin said the British proposals did "not respond to the questions the international community is asking".
Russia insisted it would veto any resolution which paved the way for military intervention in Iraq, but appeared to leave space for compromise.
Speaking on Thursday, Russia's Deputy Foreign Minister Yuri Fedotov said Iraq would need a "certain amount of time" to meet the tests presented by Britain.
Analysts say that some UK and US officials will consider it a victory if the text gets the nine votes required for approval - even if France or Russia veto it as they have threatened to do.
The six "undecided" non-permanent members of the Council - Angola, Cameroon, Guinea, Pakistan, Chile and Mexico - have yet to respond to the UK plans.
But they have previously expressed concerns about the draft resolution, which originally set a 17 March deadline for Iraq to disarm.
According to the BBC's UN correspondent Greg Barrow, privately many diplomats say that discussions in the corridors are less about disarming Iraq and more about saving Mr Blair's skin.