US aircraft and artillery hammered the rebel-held Iraqi city of Falluja on Thursday, as residents awaited an expected all-out assault.
US marines are poised to strike at Falluja
Fresh barrages were reported in the evening, following a day in which the US military said it had pounded "barricades" set up by insurgents.
US soldiers have also clashed with militants in Ramadi, west of Falluja.
US and Iraqi government troops have threatened an assault on Falluja unless insurgents based there surrender.
Forces have been positioned on the outskirts of the city for several weeks, and US planes have been carrying out regular bombing raids.
Iraqi Prime Minister Iyad Allawi and Washington say Falluja is hosting foreign fighters allied to al-Qaeda - an accusation denied by local leaders.
On Thursday night, Reuters reported that insurgents were returning US barrages from the edge of the city with fierce mortar and rocket-propelled grenade attacks.
The heaviest bombing on Wednesday night was concentrated on the eastern and north-western areas of the city, Reuters reported.
It quoted a hospital doctor saying five civilians - a woman and child among them - died as they tried to flee the bombardment in a car.
Many of Falluja's estimated 300,000 residents have already fled, reports say.
British troops from the Black Watch battalion - who recently left southern Iraq for deployment nearer Baghdad - are policing roads in the area in an effort to block supply routes to the rebels.
The US said on Thursday that the Iraqi government was putting together a shadow administration to run Falluja once the fighting was over, AP news agency reported.
Maj Jim West said the plan was to "isolate" Falluja to prevent insurgent forces from entering or leaving
He said the US was wary of guerrilla tactics in the city, like rigging buildings with explosives as booby-traps.
He said preparations were also being made to care for an exodus of people during fighting.
"We're not going to let the elderly and the infirm wander around the desert," he was quoted as saying by AP.