US forces have defended their bombardment of insurgent positions in the rebellious Iraqi city of Falluja.
American tactics in Falluja have been criticised
Commanders of besieging US marines say the assault was in response to breaches of a ceasefire.
In one of their most intensive uses of firepower, overnight artillery barrages were accompanied by the deployment of a heavily armed AC-130 gunship.
Fighting resumed on Wednesday, with reports of gunfire, mortar explosions and US military jets bombing the city.
In a statement, the US commanders said the fighting began with an insurgent attack on marine positions.
"Marines responded by directing precision weaponry against enemy forces in order to defend themselves," it said.
They added that the insurgents had failed to surrender their heavy weapons by the latest deadline set by the US for them to do so.
The US military operation began on 5 April following the gruesome killings of four American civilian contractors in the predominantly Sunni Muslim city.
Falluja lies 50 km (30 miles) west of Baghdad and has been a hotbed of armed opposition to the US-led occupation of Iraq.
Local doctors say that since then, hundreds of civilians have been killed.
Tuesday night's bombardment was shown live on television networks around the world, including al-Jazeera, which is seen widely in Iraq and throughout the Arab world.
Over the pictures of the American attack, al-Jazeera broadcast an interview with a cleric in Baghdad, who commented that the Americans were targeting ordinary civilians and that they were trying to destroy everything.
The BBC's Jennifer Glasse says such broadcasts are likely to inflame anti-American sentiments in Iraq.
US administrators say they want to avoid an all-out assault on Falluja.
Joint patrols by US marines and Iraqi security forces are seen as an alternative way of restoring control in the city. Tuesday saw the marines begin training Iraqi security
personnel for such duties.
Our correspondent, who is with the US marine force at Falluja, reports that a spokesman said it could be six to eight weeks before the Americans were in full control of the city.
General Richard Myers, chairman of the US Joint Chiefs of Staff, said on Tuesday that he was still hopeful of a ceasefire and negotiated settlement to the stand-off.
But he added: "We don't have unlimited patience."
American tactics in Falluja have been criticised by normally pro-US Iraqi politicians, as well as the United Nations special envoy to Iraq, Lakhdar Brahimi.
Mr Brahimi warned that they were threatening moves to return sovereignty to Iraqis and create a viable state.
"Unless this standoff is brought to a resolution through peaceful
means, there is great risk of a very bloody confrontation... the consequences of such bloodshed could be dramatic and
John Negroponte, who has been named as US ambassador to Iraq after the
handover of power to an interim government on 30 June, dismissed such concerns.
"There's no place for armed groups of this kind in the future of
Iraq and they must be dealt with," he told reporters.
"Sometimes force has to be met by force."
The UK government has defended the action of US troops in Falluja.
Prime Minister Tony Blair's official spokesman said: ''The insurgents launched an attack on troops, and if they're attacked they have the right to respond.''
He added: ''We'd like this resolved politically, but that's only possible if there is the will on the other side.''