At least five people have been killed in clashes between gunmen and police in the Iraqi town of Baquba, north of Baghdad, Iraqi and US officials say.
There has been a surge in attacks on US troops in recent months
The fighting has been going on for several hours and has spread through the town, the US military say.
The police have called for support from the Iraqi army.
Elsewhere, four marines and a sailor were killed in fighting in Anbar province, west of Baghdad, on Wednesday, the US military said.
Baquba lies in a mixed Sunni and Shia province where attacks by Sunni insurgents against the Iraqi security forces have been common.
The Americans say they have not intervened so far.
The US personnel in Anbar province died on Wednesday "from injuries sustained due to enemy action", a statement said.
More than 90 US personnel have died in Iraq so far this month - the highest monthly toll since November 2004.
US President George Bush on Wednesday admitted being seriously concerned about the scale of American casualties.
However, he ruled out a fixed timetable for the withdrawal of troops.
His comments came two weeks ahead of crucial US mid-term elections, and amid public unease over US policy in Iraq.
Three hundred Iraqi troops have also died in October.