The US has rejected the United Nations secretary-general's claim that the US-led invasion of Iraq was illegal.
Kofi Annan told the BBC the decision to take action in Iraq contravened the UN charter and should have been made by the Security Council, not unilaterally.
But US State Department spokesman Richard Boucher said the use of force against Iraq had a sound legal basis.
Earlier, US allies including the UK, Australia and Poland also said the war was backed by international law.
In Baghdad, the interim Iraqi government said Iraqis had been happy to see the end of Saddam Hussein.
'We don't agree'
Mr Boucher said military action was consistent with UN resolutions which referred to "serious consequences" if Iraq failed to comply with UN demands.
"While we respect his views I think we have made it clear we don't agree," he said.
President Bush himself has not directly responded to Mr Annan's comments.
But while campaigning in Minnesota for November's presidential election, he reiterated the US position that Security Council resolution 1441, passed in November 2002, gave the invasion legal force.
"The United Nations looked at the same intelligence I looked at," AFP news agency quoted Mr Bush as saying. "They concluded Saddam Hussein was a threat. They voted by 15-0 in the UN Security Council for Saddam Hussein to disclose, disarm or face serious consequences."
Mr Annan's criticism has particular political resonance in the US, where the Bush administration's performance on Iraq is a key issue in the election, the BBC's Jill McGivering reports from Washington.
Mr Annan's disapproval of the US-led action was already well known. But this is the first time he has used the word "illegal", our correspondent says.
Authorities in the UK, Australia, Poland, Bulgaria and Japan also rebuffed Mr Annan's claims.
Australian Prime Minister John Howard described the UN as a "paralysed" body and said it was incapable of dealing with international crises.
"The legal advice we had - and I tabled it at the time - was that the action was entirely valid in international law terms," he said.
The British government - which has argued that UN resolutions provided a legal basis for intervening to topple Saddam Hussein - said the 2003 invasion was "not only lawful but necessary".
Japan's top government spokesman, Hiroyuki Hosoda, told a news conference that he would be seeking clarification about the exact significance of Mr Annan's words.
Polish Foreign Ministry spokesman Boguslaw Majewski said the "decisions which at that time were made by the international community in Iraq, did have legal basis".
And Bulgarian spokeswoman Guergana Grantcharova cited previous Security Council resolutions which, she said, supported the case for war.
Mr Annan told the BBC World Service there should have been a second UN resolution following Iraq's failure to comply over weapons inspections.
He also warned security in Iraq must considerably improve if credible elections are to be held in January.