US warplanes and tanks have been bombarding areas around the Imam Ali shrine in the Iraqi city of Najaf.
The latest bombardment started late on Thursday evening
Radical Shia cleric Moqtada Sadr has continued to defy demands to end his insurgency, despite a "final call" from Iraqi Prime Minister Iyad Allawi.
Orange flashes lit up the night sky as US warplanes flew overhead and thick black smoke rose from the old city.
The fighting died down a few hours before dawn, but there as yet is no word on casualty figures.
The BBC's Kylie Morris in Najaf said the bombardment by US planes was intense, leaving a smoky haze that stretched across the night horizon.
The bombing went on for more than 30 minutes, and the explosions could be heard several kilometres away.
A large cloud of smoke was seen rising near the Imam Ali shrine, from where Sadr supporters have been fighting US and Iraqi forces.
More than 1,000 Sadr supporters are in the Imam Ali shrine
A letter, purportedly from Mr Sadr, said he would turn the shrine over to senior clerics, who would also decide the fate of his Medhi army militia.
"I call on the religious authority again to receive the
shrine so that it won't be taken by the hands of the enemy
and of treason," the letter read.
However, its authenticity has not been confirmed, and Mr Sadr is still refusing to disband his militia.
In other developments in Iraq:
- American warplanes fire missiles at targets in the volatile city of Falluja, west of Baghdad
- Suspected Sadr militants set fire to the headquarters of Iraq's South Oil Company in Basra
- The US embassy in Iraq is hit by mortar fire, slightly injuring two employees
- A new US army report on the Abu Ghraib prisoner abuse scandal will implicate at least two dozen more personnel, say US defence officials, and a report by an American academic says some medics collaborated with abusive guards.
On Thursday, Prime Minister Allawi told a news conference in Baghdad that Mr Sadr's fighters had to "disarm, vacate the holy shrine, engage in political work and consider the interests of the homeland".
Mr Allawi said he welcomed an offer from Mr Sadr to end his uprising - made on Wednesday to the Iraqi national conference that ended its work by appointing a national council to oversee the government's work.
"We welcome such an announcement and confirm our readiness to accept this initiative of his provided that he crystallises it into a tangible and committed position through a declaration from him personally," he said.
The prime minister declined to say whether there was a deadline for Mr Sadr to dismantle his militia and leave the holy sites.
US troops are deployed around the shrine
But the crisis is being seen as a crucial test for the authority of the interim Iraqi government.
It knows that it would be political suicide for Mr Sadr to personally condemn the violence - having invested so much of his political capital in his militia, says the BBC's Arab affairs analyst Magdi Abdelhadi.
The government has concluded that if it is too dangerous to kill Mr Sadr - and thus turn him into a martyr - it can at least try to destroy him politically, our analyst says.