US troops have launched a big offensive against Iraqi insurgents in the Sunni flashpoint town of Falluja.
Hundreds of US and Iraqi troops have surrounded the city
They have sealed off the town where four Americans were killed and their bodies mutilated by a mob last week.
Unconfirmed reports say several people were killed when a US warplane dropped bombs on a residential area of the town after a mortar attack on troops.
The US command has vowed to "pacify" Falluja, where four civilian contractors were killed on Wednesday.
Falluja lies in the heartland of resistance to the US-led occupation.
US troops closed off entrances to Falluja with earth barricades ahead of the planned operation, code named "Vigilant Resolve".
About 1,200 marines and two battalions of Iraqi security forces were poised to enter the city to arrest suspected insurgents, Associated Press news agency quoted Lt James Vanzant of the 1st Marine Expeditionary Force as saying.
"The city is surrounded," Mr Vanzant said.
"It's an extended operation. We want to make a very precise approach to this. ... We are looking for the bad guys in town."
US aircraft were reported to have struck a residential area in the city, killing many people according to witnesses.
There has been no confirmation by the US-led coalition.
The operation was to last several days, the US military said.
The four US security guards - working for Blackwater Security Consulting - were ambushed and killed as they drove through Falluja last Wednesday.
An angry crowd surrounded the cars, throwing stones and setting the vehicles ablaze.
28 April 2003: US paratroopers shoot dead 13 demonstrators
May 2003: Grenade and gun attacks on US troops become a routine occurrence
November 2003-January 2004: Three helicopters are shot down in the area with the total loss of 25 lives
31 March 2004: Four US contractors killed and mutilated
The corpses were dragged from the wreckage and television pictures showed one burnt body being kicked and stamped on, while at least two were tied to cars and driven through the streets, witnesses said.
Adults and children hacked the bodies to pieces, before lynching two of the charred remains from a bridge spanning the Euphrates River.
Two of them were hanged from a bridge.