Iraqi security forces are battling with powerful Shia militias in a major operation to bring Basra under government control.The operation targeted six districts in central and northern Basra where the Mehdi Army militia of Shia cleric Moqtada al-Sadr has a strong presence. And violence appeared to be spreading to Baghdad and other cities.Police and health workers said at least 12 people were killed in the fighting in Basra.One resident said: "There are clashes in the streets. Bullets are coming from everywhere and we can hear the sound of rocket explosions. This has been going on since dawn."Columns of black smoke rose above the city and explosions and machinegun fire could be heard. TV pictures showed masked gunmen firing mortars in the street, while others drove around in captured Iraqi army and police vehicles.The Mehdi Army, which has thousands of fighters, has kept a relatively low profile since last August when Sadr called a ceasefire, one of the main factors behind the sharp reduction in sectarian violence in Iraq in recent months.But the militia has broken the truce, saying US and Iraqi forces exploited it to carry out indiscriminate arrests.Sadr called on Iraqis to stage sit-ins all over Iraq and said he would declare a "civil revolt" if attacks by US and Iraqi security forces continued.Sadr's followers launched what they called "a civil disobedience campaign" in Baghdad on Monday, forcing store-owners to close in several districts. And pro-Sadr students forced Mustansiriya University in Baghdad to close.Members of Sadr's movement said the protest would spread to other towns and cities from Wednesday.Police sources said Sadr supporters seized control of five districts in the southern town of Kut on Tuesday after clashes between gunmen and police.In Baghdad, US and Iraqi forces sealed off the Mehdi Army stronghold of Sadr City, a sprawling slum of 2 million people, after the militia ordered police and soldiers off the streets.Police said fighting erupted in several Sadr City neighbourhoods between Mehdi Army fighters and the Badr Organisation, the armed wing of a rival Shia faction.Baghdad's Green Zone, the government and diplomatic compound, was hit by several salvoes of rockets during the day. US military spokesman Lieutenant-Colonel Steven Stover said they had been fired from Sadr City.Police have also imposed curfews in the southern towns of Kut, Hilla and Samawa, capital of Muthanna province.Major Tom Holloway, a spokesman for British forces in Basra, said Iraqi Prime Minister Nuri al-Maliki was in the city to oversee the operation."It will be very difficult for the central government to regain control," said Joost Hiltermann, an Istanbul-based analyst for the International Crisis Group think-tank. "You have many armed groups that are looking to keep hold of their share of the oil wealth."Sadrists and the Supreme Islamic Iraqi Council, the two most powerful Shia factions in Iraq, have been vying for control of Basra, Iraq's second city and gateway to the Gulf, along with a smaller Shia party, Fadhila.Iraqi army Major-General Ali Zaidan, the commander of Iraqi ground forces in the operation, said the offensive would continue "until we achieve our target"."The target is to wipe out all the outlaws. There were clashes and many outlaws have been killed," Maj-Gen Zaidan said© Independent Television News Limited 2008. All rights reserved.