Three Americans were seriously injured in the attacks on Wednesday, US Embassy spokeswoman Mirembe Nantongo said. At least four Iraqis also were killed after at least two mortar or rocket rounds fell short in Shiite areas of Baghdad.
A resumption of intense fighting by al-Sadr's Mahdi Army militia could kill more US soldiers and threaten — at least in the short run — the security gains Washington has hailed as a sign that Iraq is on the road to recovery.
The burgeoning crisis — part of an intense power struggle among Shiite political factions — also will test the skill and resolve of Iraq's Shiite-led government in dealing with Shiite militias, with whom the national leadership had maintained close ties.
The Sadrists are angry over recent raids and detentions, saying US and Iraqi forces have taken advantage of the cease-fire to crack down on the movement.
They also have accused rival Shiite parties, which control Iraqi security forces, of engineering the arrests to prevent them from mounting an effective election campaign. The showdown with al-Sadr has been brewing for months but has accelerated since parliament agreed in February to hold provincial elections by the fall.
On Wednesday in Basra, gunfire echoed through the streets as Iraqi soldiers and police fought the Mahdi Army, police said.
Reinforcements were sent to Basra from the Shiite holy city of Karbala, Interior Ministry spokesman Maj. Gen. Abdul-Karim Khalaf said, adding a large number of gunmen have been detained.
Mortar rounds also hit a detention center in central Basra and injured 10, police said.
Sadiq al-Rikabi, a chief adviser to al-Maliki, said gunmen who fail to turn over their weapons to police stations in Basra by Friday will be targeted for arrest. He added that they also must sign a pledge renouncing violence.
"Any gunman who does not do that within these three days will be an outlaw," he said.
Iraqi officials say at least 40 people were killed and 200 wounded in the fighting in Basra, Iraq's second-largest city, 340 miles southeast of Baghdad.
British troops have remained at their base at the airport outside Basra and were not involved in the ground fighting, although British planes were providing air surveillance, according to the British Ministry of Defense. It said Wednesday that the Iraqis had not asked the British to intervene.