Iraqi security forces are reported to have lost control of large parts of the northern city of Mosul.
Insurgents attacked several police stations in the city
Masked insurgents overran and set fire to several police stations, looted weapons and ammunition and roamed the streets, setting fire to police cars.
The US military said that it had responded to the upsurge in attacks in the city with both air and ground fire.
Correspondents say Mosul has seen a spate of suicide car bombings against Iraqi and US forces in recent weeks.
'Insurgents are everywhere'
As darkness fell, the sound of explosions and gunfire
echoed through the centre of Iraq's third-largest city, AFP news agency reports.
In a statement, the US military said that it had launched an operation in southern Mosul to try to quell the insurgency after a request from the governor.
"Insurgent forces attacked several police stations and other
targets within the city," it said.
"In several cases, anti-Iraqi
forces exceeded the capabilities of the police on site,
Several insurgents were killed in the strikes, a US military spokeswoman told AFP, without giving numbers.
The city's governor imposed an indefinite curfew on Wednesday after militants killed four members of the Iraqi security forces and a foreign contractor.
"It doesn't feel like the police or any local government
officials are in charge at all," a resident told Reuters news agency.
insurgents are everywhere."