Iraqi rebels are patrolling parts of Mosul following an upsurge of violence in the northern city, eyewitnesses say.
A car bomb is said to have targeted Iraqi reinforcements in Mosul
"They are guarding hospitals, schools and fire stations," one resident told Reuters news agency.
Other reports say gunmen are holding the governor's office and other key buildings - but US-led forces insist they still control the city.
The Baghdad government sent reinforcements to Mosul after police stations there were overrun by rebels.
And the US army is reported to have diverted 500 soldiers from the fighting in Falluja and sent them to Mosul.
Brig Gen Carter Ham, commander of American forces in Mosul, said US forces had expected "some reaction" in the city in response to the US-led assault on Falluja.
But he told US news network CNN he doubted the militants had come to the northern city having escaped from Falluja.
Most "were from the northern part of Iraq, in and around Mosul and the Tigris river valley that's south of the city," he said.
The interim Iraqi government fired Mosul's police chief after nine police stations fell into the hands of insurgents on Friday.
An official of the Patriotic Union of Kurdistan party alleged that many police in the city were co-operating with the militants.
The party's headquarters was attacked in a gun battle that left six fighters dead, he told the Associated Press news agency.
Gunmen attacked several police stations and looted weapons
Friday also saw insurgents assassinate the head of Mosul's anti-crime task force, Brig Gen Mowaffaq Mohammed Dahham, and torch his home, AP reported.
The US carried out air strikes on Thursday night after a rise in attacks in the city.
The US military said it had launched its operation after a request of the city's governor.
He imposed an indefinite curfew on Wednesday after militants killed four members of the Iraqi security forces and a foreign contractor.
Mosul has seen a spate of suicide car bombings against Iraqi and US forces in recent weeks.