However, the White House said it would not request an immediate recall of the Security Council to discuss possible military action. Correspondents say the US allegations against Iraq could be challenged by other UN members.
Iraq denied that it had breached the resolution, and accused the United States of itself violating international law by implementing no-fly zones across its territory and using the UN as a mask for its aggression.
The new war of words erupted as UN weapons inspectors said they were making progress in talks with Iraqi officials after they arrived in Baghdad with a sweeping new mandate to search the country for weapons of mass destruction.
'Unacceptable'
US and British warplanes attacked Iraqi air-defence targets for the second day in a row on Sunday because they had been "threatened", US military chiefs said.
US Defence Secretary Donald Rumsfeld said Iraqi firing at the planes was "unacceptable".
White House spokesman Scott McClellan said the attack on aircraft patrolling near the northern town of Mosul "appears to be a violation" of the resolution agreed unanimously by the UN Security Council earlier this month.
He said the US therefore had the option to demand a recall of the Security Council to discuss further action against Iraq, but neither he nor Mr Rumsfeld said that was an immediate possibility.
Mr Rumsfeld said: "It is for the president of the United States and the UN Security Council to make judgements about their view of Iraq's behaviour over a period of time."
Mr McClellan added that the resolution said Iraq "shall not take or threaten hostile acts" at anyone upholding UN directives.
Iraq's reaction was swift and angry.
"This US declaration is an additional expression of American intentions to use [UN] resolution 1441 as a cover to justify its aggressive actions against Iraq," a foreign ministry spokesman was quoted by the official INA news agency as saying.
A BBC correspondent at the UN, David Bamford, says the US interpretation of resolution 1441 to include protection for aircraft patrolling the no-fly zones could prompt new debate about what exactly was agreed in the Security Council.
Our correspondent says many people believe Iraq's compliance with the resolution would be judged solely on how the inspectors are handled.
Mr Blix and French Foreign Minister Dominique de Villepin are among those who have said Iraqi actions regarding the no-fly zones would not be a "material breach" of the agreement, he says.
Leaflet drops
Baghdad has long disagreed with the no-fly zones which were imposed by the US, Britain and France after the 1991 Gulf War in what was described as a humanitarian effort to protect Shia Muslims in the south and Kurds in the north.
US planes have also dropped leaflets calling on Iraqi forces not to fire on aircraft, warning "You could be next" for retaliatory attacks.
Our correspondent in Washington, Justin Webb, says the Bush administration believes that sooner or later the inspectors will be blocked.
In the meantime, the US military is continuing its build-up in the region, he says.
Next steps
18 Nov: Inspectors arrive in Iraq
8 Dec: Iraq must reveal all programmes, plants and materials which could be used for weapons production
23 Dec: Inspections must have resumed
21 Feb: Inspectors to report to UN Security Council
In Baghdad, talks have begun between officials and international inspectors who returned to Iraq for the first time in four years.
The leader of the UN team, Hans Blix, said inspections would resume on 27 November.
After the first meeting with government officials, he said he was "making progress".
The UN resolution requires Iraq to declare all its weapons programmes by 8 December.
It also gives inspectors powers to check any site in Iraq.