In another attack, a car bomb killed at least five people and wounded 10 more on Tuesday near Baghdad University, Iraq's biggest and oldest institute of higher education, security officials said.
The attack took place near the university campus in the southwestern Jadiriyah district of the capital.
In central Baghdad, clashes erupted between US forces and fighters and an American helicopter came under ground fire but was not brought down, US and Iraqi officials said on Tuesday.
"The chances of success [in Iraq] are essentially zero because the Iraqi people have no voice"
Non Sequitur, Cadiz, Spain
Send us your views
The fighting raged in Fadhil and Sheikh Omar, two Sunni-majority neighbourhoods, two Iraqi police officials said.
A steady rattle of gunfire reverberated across the centre of Fadhil. An Iraqi defence ministry official said one person was killed and 15 others wounded in the clashes.
US military helicopters could be seen flying over the district.
More US deaths
Three American soldiers have been killed and another has been wounded after a roadside bomb exploded near their patrol in Baghdad, the US military announced on Tuesday.
In a statement, the military said the incident took place in southeastern Baghdad.
The latest deaths bring to around 40 the number of US soldiers killed in Iraq this month, putting April on course to be one of the deadliest months for US soldiers in some time.
More than 3,280 American soldiers have been killed in Iraq since the 2003 invasion.