Iraq's prime minister on Wednesday gave gunmen in the southern oil port of Basra a three-day deadline to surrender their weapons and renounce violence as clashes between Shiite militia fighters and Iraqi security forces erupted for a second day. At least 55 people were killed and 300 wounded in Basra and Baghdad after the fighting spread to the capital's main Shiite district of Sadr City, according to Iraqi police and hospital officials. The ultimatum came as Prime Minister Nouri al-Maliki was in Basra to supervise a crackdown against the spiraling violence between militia factions vying for control of the center of the country's vast oil industry located near the Iranian border. Suspected Shiite extremists also unleashed rockets or mortars against the US-protected Green Zone in central Baghdad for the third day this week. 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. The violence has raised fears that the cease-fire declared in August by al-Sadr could unravel, presenting the gravest challenge to the Iraqi government in months. A resumption of intense fighting by his 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 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. Tensions continued Wednesday in Basra, the center of the country's vast oil industry located near the Iranian border. 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 would 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, 550 kilometers 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. British forces turned over responsibility for Basra to the Iraqis in late December but say they will assist the Iraqis upon request. Fifteen people were killed and 100 wounded in clashes in Sadr City that broke out Tuesday, according to police and hospital officials, who spoke on condition of anonymity because they weren't authorized to release the information. Hundreds of Sadr City residents took to the streets on Wednesday, demanding the government stop military operations in Basra and other cities and withdraw all security forces. "We strongly condemn the assaults being conducted by the occupation forces along with the Iraqi security forces who have sold themselves to the renegade occupier," demonstration leader Sheik Saleh al-Eraibi.