There has been fighting between US troops and insurgents in the Iraqi capital and the city of Ramadi.
Insurgents staged a coordinated attack in Ramadi
US troops were involved in a seven-hour clash in Baghdad's Adhamiya district after insurgents attacked Iraqi security forces.
In Ramadi, insurgents launched a coordinated attack on a government building, US military officials said.
Earlier, the bodies of 12 men were found in Baghdad, Iraq's interior ministry said.
A military spokesman said US troops took part in fighting in the Sunni Muslim-dominated Adhamiya district in the north of Baghdad.
Troops went in to provide support to Iraqi security forces after about 50 insurgents attacked. The spokesman described it as "quite a battle".
He said five rebels were killed and one member of the Iraqi security forces was injured. There were no US casualties, he said.
'Multiple car bombs'
"Closely coordinated" attacks in Ramadi were also repulsed, the US military said.
A statement said attacks on the main government building and other sites involved "multiple homicide car bombs, mortars, rocket-propelled grenades, and heavy machine gun and small arms fire".
US Marine Capt Andrew Del Gaudio told the Associated Press news agency that troops fired on two suicide car bombers heading for the government building.
Other insurgents fired at Marine positions on the roof of the building and at another observation post, he said.
It was not clear if there were casualties in the clash.
In other violence, a roadside bomb targeted an army patrol in central Baghdad, killing a civilian and wounding several others, police said.
This comes as the bodies of 12 men were found in Baghdad.
The identities of the victims were not known, but officials said all were shot and some showed signs of torture.
Iraq is going through a wave of sectarian and anti-US violence
Hundreds of bodies have been recovered in the past two months in Iraq, in a spate of sectarian killings since a bomb attack on a key Shia shrine.
Earlier, security officials said they had found the body of the brother of a prominent Sunni Arab politician.
Taha al-Mutlaq, a businessman, was kidnapped several weeks ago, and found shot in the head.
He was the brother of Saleh al-Mutlaq, a leader of the Iraqi Front for National Dialogue.
The violence comes as talks are stalled on a new national unity government.
Sunni, Kurdish and secular parties say they will not be partners in an administration headed by the current Shia-nominated Prime Minister Ibrahim Jaafari.