Fighting sparked by a government-led push against "outlaw" militias in the southern city of Basra had left more than 280 people dead by March 29, according to Iraqi authorities.

The unrest has stretched across southern Iraq's Shiite heartland up to Baghdad, where a ban on pedestrian and vehicle traffic was kept in place just hours before it was due to expire on Sunday morning.

US warplanes and British artillery struck targets in Basra on Saturday. Another Basra airstrike killed 16 "criminal fighters," and a joint US-Iraqi patrol killed 13 more fighters in southeastern Baghdad's Suwayrah district, US commanders reported.

The violence has sparked fears that a seven-month cease-fire by the Mehdi Army, regarded as a key factor in a dramatic drop in attacks in recent months, could collapse or that the US military will have to bail out the Iraqis.

Prime Minister Nuri al-Maliki met on Saturday in Basra with the area's leaders, who have expressed support for the government's efforts to "impose law and save Basra from criminal gangs," according to a written statement from the prime minister's office.