US forces have swept through parts of the northern Iraqi city of Mosul in an operation to flush out insurgents and retake captured police stations.
US troops began their operation early on Tuesday
More than 1,000 soldiers are advancing through the city from west to east, meeting little resistance, the US says.
An overnight curfew is in force and the city's five bridges have been closed.
Mosul, Iraq's third largest city, descended into lawlessness last week as US forces began a major operation to take control of the city of Falluja.
Insurgents stepped up attacks in Mosul, seizing or destroying several police stations. They also looted police uniforms and vehicles, making it even harder for the Americans to know friend from foe.
The city's police chief, Brig Gen Mohammed Kheiri Barhawi, was fired amid reports that some police co-operated with insurgents.
The BBC's Caroline Hawley in Baghdad says the insurgents are trying to stretch the US military by stepping up their attacks in several parts of the country.
She adds that they seem to be trying to avenge the humiliation of a foreign army taking over an Iraqi city.
All the police stations in west Mosul have been retaken, a US officer said late on Tuesday, and the same process would start on the east side of the city.
"We are in the process of securing all of the police stations and returning the police to these stations to [put] in place a strong police presence," said a US military spokeswoman.
A journalist in Mosul, Rouaa Zarari, says the situation is now largely calm, although there is a heavy troop presence on the streets.
The insurgents seem to have melted away, she told the BBC.
In Falluja, US-led forces say they have now gained overall control, trapping rebels in the south of the city.
However, the military reported that a US soldier was killed in a car bomb at a checkpoint in the city on Tuesday.
In other developments:
- The family of Dublin-born Iraqi aid worker Margaret Hassan say they believe she has been killed by hostage-takers
- New clashes break out between US-led troops and insurgents in Baquba, following fighting there on Monday
One person dies and three are injured in a rocket attack on a commercial district of the northern city of Kirkuk, AP says
- One US soldier is killed and one wounded in an attack on a convoy near Balad, north of Baghdad
- The US military charges an officer with the killing of an injured Iraqi in August - two other soldiers already face murder charges over the incident
The operation in Mosul began early on Tuesday, with US warplanes and helicopters flying overhead as loud explosions and gunfire were heard.
The streets of Mosul, which has a population of around a million, were deserted as the curfew began at 1800 (1500GMT).
The BBC's diplomatic correspondent, Jonathan Marcus, says that while the insurgents appear to have suffered serious losses in Falluja, it is clear that the US simply does not have sufficient troops on the ground to maintain order in several key cities at once while launching a major offensive against another.