The U.S. military said the overnight raids were carried out in Al-Amarah, the capital of Maysan Governorate, and the nearby town of Al-Majar al-Kabir.
Iraqi police said the clashes involved the Al-Mahdi Army, the militia commanded by radical Shi'ite cleric Muqtada al-Sadr.
British forces handed back security control of Maysan, a predominantly Shi'ite region that borders Iran, to Iraqi security forces in April.
Elsewhere today, a policeman and a militant were killed in clashes that erupted between Iraqi police and Al-Mahdi Army fighters in Al-Nasiriyah, about 120 kilometers south of Al-Amarah.
(Reuters, AP, AFP)