At least 28 people have died in fighting between Iraqi police and gunmen in Baquba, 65km north of the capital, Baghdad, officials say.
There has been a surge in violence in recent weeks
US military sources say the fighting went on for a number of hours, spreading to several town districts.
At least 12 of the dead were Iraqi police, and 35 people were injured.
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, though US military officials say they do not know if those involved in this incident are Sunni or Shia.
The US personnel who were killed in Anbar province on Wednesday died "from injuries sustained due to enemy action", a US military 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.
Sadr City raid
In a separate development, Iraqi Prime Minister Nouri al-Maliki has said a raid by US and Iraqi forces in a mainly Shia area of Baghdad on Wednesday was aimed at Abu Deraa - a notorious death squad commander believed to be behind a number of sectarian killings.
In an interview with the Reuters news agency, Mr Maliki said the raid in Sadr City had not succeeded in capturing the commander, though 13 people were arrested.
On Wednesday Mr Maliki appeared to say that he had not been informed about the raid, but in the interview he said that he had backed it.
Mr Maliki accused the US of heavy-handed raid tactics
However, he complained that it had been carried out in a heavy-handed way, which had the potential to wreck a deal he had brokered with Moqtada al-Sadr, the cleric who leads the Shia militia called the Mehdi army.
"Do you send in planes to arrest one person?" Mr Maliki asked.
The US military has said 10 enemy fighters were killed in the operation, which at one point involved an attack by US aircraft, called in to back up Iraqi special forces.
Mr Maliki said that problems stemmed from the fact that the raid had been combined with attempts to find a captured US soldier.
A spokesman said the military was reviewing its procedures for informing the Iraqi government about operations.