The military said Operation Swarmer, using 1500 US and Iraqi soldiers and 50 helicopters, was the biggest air assault since a similar airlift just after the war to oust Saddam Hussein, the Iraqi president, three years ago.

It involved searching a 16km by 16km area for fighters, who gather support from the Sunni community, dominant under Saddam.

US and Iraqi forces pressed on with the attack as Iraqi leaders failed again to form a coalition government widely seen as vital to avert any slide into civil war from sectarian bloodletting between Sunni Arabs and Shia Muslims.

Zalmay Khalilzad, the US ambassador to Iraq, has been mediating in the hope that Iraqi leaders will finally bury differences and form a national unity government three months after parliamentary elections.

Political sources said there may be a breakthrough in coming days, with the powerful but factionally divided Shia Alliance appearing more willing to consider an alternative to Ibrahim al-Jaafari, the Iraqi prime minister.
   
Kurdish and Sunni leaders want to see him dropped.

Unhelpful activities
    
Condoleezza Rice, the US secretary of state, told a news conference on a trip to Australia on Friday that she believed talks with Iran on stabilising Iraq would be useful.

The United States again accused Iran of meddling in Iraq's internal affairs, saying the Iran was carrying out "unhelpful activities".
   
The US embassy issued the statement a day after Iran said it had accepted a proposal by Abdul Aziz al-Hakim, an Iraqi Shia leader, to open talks with Washington on Iraq.

Sunni leaders, deeply suspicious of ties between Tehran and Iraq's Shia leaders, dismissed any such talks.

Further attacks
   
In new violence, a bomber got on to a bus and detonated his explosives belt, killing the driver and wounding four people nearby, police said.

Police said three bodies with gunshot wounds to the head and signs of torture were found in Baghdad - apparently part of the latest wave of sectarian violence that has left more than 100 corpses dumped in the capital alone since Monday.
   
South of Baghdad in Mahmudiya, in an area known as the "Triangle of Death" for its insurgent attacks, two Shia pilgrims walking to the holy city of Karbala were killed by a roadside bomb, police said.

Another roadside bomb killed a policeman in nearby Latifiya.