Policemen stand guard on a road in Basra March 25, 2008. Heavy fighting broke out between Iraqi security forces and the Mahdi Army Shiite militia in Basra on Tuesday, a local security source and witnesses said. (Xinhua/Reuters Photo)

Heavy fighting broke out between Iraqi security forces and the Mahdi Army Shiite militia in Iraq's southern city of Basra on Tuesday, a local security source and witnesses said.

Sounds of explosions and machinegun fire resonated in the city overnight and in the morning soon after the Iraqi security forces surrounded several of the city's neighborhoods which are known as Mahdi Army militia strongholds, witnesses at the scene told Xinhua on condition of anonymity.

The police confirmed that neighborhoods of Haiyaniyah, Jamhuriyah in central the city were surrounded by Iraqi security forces, as they were believed to be the main bastions of the Mahdi Army militia.