There has been a renewed exodus of residents from the Iraqi city of Falluja, after one of the heaviest US bombardments for weeks.
The strikes were carried out late overnight into Friday
Ground forces were also involved, but the US said the operation was not the start of an attempt to retake the city.
At least eight people were killed and several injured, medical sources say.
In a southern suburb of Baghdad, a car bomb exploded near a police station killing 10 people, including a family of four, the US military said.
It said the car was packed with 300 lb
(135 kg) of explosives, and blew up at about 1100 (0800 GMT) as an Iraqi police patrol passed. All the dead were civilians.
In Falluja, the US said planes hit a planning centre used by militants led by Iraq's most wanted man, Abu Musab al-Zarqawi.
During the operation, American troops reportedly arrested Sunni Muslim cleric Khaled al-Jumaili, a senior member of a delegation which had been negotiating a truce in Falluja.
Iraqi police said US marines also detained the city's police chief and two other police
officers as they moved their families to a nearby
town to escape American air raids.
Meanwhile US forces have stepped up security in Baghdad, after the first bombings inside the city's fortified Green Zone killed up to six people on Thursday.
In other developments:
- A British security guard is shot dead close to the northern Iraqi city of Kirkuk, his company confirms
Polish Prime Minister Marek Belka says Poland plans to reduce its deployment of 2,500 troops in Iraq, from January next year
Iraqi Interior Minister Falah al-Nakib tells Iraqi police cadets training in Jordan that Iraqis could be responsible for their own security in a "very short time"
The US government agrees to a special audit into the use of hundreds of millions of dollars of Iraqi oil revenues since the US-led invasion, the UN says.
'No all-out assault'
Residents of Falluja are packing their bags and leaving town fearing an imminent US ground invasion.
"They bombed us with their planes and people started
running away from home," one resident told Reuters news agency.
"The situation now is very difficult
and we are leaving now."
The US military has denied that a full assault on the city is under way.
The air strikes follow a call to residents by Iraqi interim Prime Minister Iyad Allawi to give up Zarqawi, a Jordanian-born militant.
But the city delegation said the government's demand was "impossible" to fulfil.
US warplanes and helicopters launched sustained attacks on Falluja late on Thursday, reportedly leaving at least five people dead.
Two more air strikes were launched on Friday morning. Medical sources said at least three people had died.
'Targets of opportunity'
At least 1,000 US ground troops were advancing towards the city with tanks and Iraqi special forces, the AFP news agency said.
Marines spokesman Lyle Gilbert told the agency the mission was to disrupt the
militants' ability to conduct terror attacks in the area.
The US military said the air strikes targeted illegal checkpoints, a weapons cache and safe houses belonging to Zarqawi's group.
The attacks came hours after leaders from Falluja suspended peace talks with officials from Iraq's interim government.
The US and the Iraqi interim government have been trying to regain control of rebel areas before elections planned for January, staging weeks of what they call "precision strikes" against militants in the city.
Zarqawi is thought to be based in Falluja. The US has offered a $25m reward for his capture.
His group - which the US says is linked to al-Qaeda - has captured and killed hostages. It also said it carried out Thursday's twin bombings in Baghdad's Green Zone in Baghdad.