A medical source said the bodies of eight gunmen were brought to the hospital.
The US military said it was looking into the report.
Elsewhere, assassins killed a police official and an aide to Iraq's pre-eminent Shiite cleric.
The new bloodshed came a day after followers of Shiite cleric Moqtada al-Sadr pushed through a resolution in Iraq's parliament requiring the Baghdad government to obtain parliament's approval for future extensions of the UN mandate for US-led forces in Iraq.
The present mandate doesn't expire until Dec. 31, but Tuesday's action added to the debate.
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