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BAGHDAD - Iraq's prime minister on Wednesday gave gunmen in the southern oil port of Basra three days to surrender their weapons and renounce violence as clashes between security forces and Shiite militia fighters erupted for a second day.

Suspected Shiite extremists also unleashed rockets or mortars against the U.S.-protected Green Zone in central Baghdad for the third day this week.

Three Americans were seriously injured in the attacks, U.S. Embassy spokeswoman Mirembe Nantongo said. At least eight Iraqis also were killed after rounds fell short in several areas of Baghdad.

At least 55 people have been killed and 300 wounded in Basra and Baghdad after the fighting spread to the capital's main Shiite district of Sadr City, police and hospital officials said.

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.

The violence has raised fears that the cease-fire declared in August by Shiite cleric Muqtada al-Sadr could unravel, presenting the gravest challenge to the Iraqi government in months.

Sadiq al-Rikabi, a chief adviser to al-Maliki, said gunmen in Basra who turn over their weapons to police stations and sign a pledge renouncing violence will not face prosecution.

"Any gunman who does not do that within these three days will be an outlaw," he said.

A resumption of intense fighting by al-Sadr's Mahdi Army militia could kill more U.S. 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.

Officials in al-Sadr's headquarters in Najaf, speaking on condition of anonymity because of the sensitivity of the issue, said the anti-U.S. cleric had sent local representatives to ask al-Maliki to leave Basra and resolve the problems peacefully. The aides also told the government no negotiations could be held until Iraqi reinforcements withdrew from the city.

The burgeoning crisis -- part of an intense power struggle among Shiite political factions -- will test the skill and resolve of Iraq's Shiite-led government in dealing with Shiite militias, which have close ties with the national leadership.

The Sadrists are angry over recent raids and detentions, saying U.S. and Iraqi forces have taken advantage of the cease-fire to crack down on the movement.

They 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.

U.S. military spokesman Maj. Gen. Kevin Bergner insisted the Sadrists were not being singled out and blamed Iranian-backed rogue militia factions for the recent violence, although he declined to link Iran directly to the fighting.

Bergner also noted the Iraqi government was taking the lead in the Basra operation, although U.S. troops were involved as members of transition teams helping train the Iraqis.

He said the Iraqi government had appealed to Iran to help restore calm in Basra.

"This is not a battle against the (Mahdi Army) nor is it a proxy war between the United States and Iran," he said. "It is the government of Iraq taking the necessary action to deal with criminals on the streets."

The White House welcomed al-Maliki's actions in Basra.

"This is exactly what we all want to see, which is the government of Iraq taking the initiative that was afforded to it by the surge and going aggressively after illegal criminal gangs and illegal militias in the Basra region," said White House spokesman Tony Fratto.

Gunfire echoed through the streets of Basra as Iraqi soldiers and police fought the Mahdi Army, police said.

AP Television News video showed militia members in fierce street battles with security forces. Tanks rumbled in the background as gunmen exchanged rocket-propelled grenade and machine gun fire. They also danced in the streets and around burned out Iraqi military vehicles.

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 hit a detention center in central Basra and wounded 10, police 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 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.

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.

Sheik Salman al-Feraiji, al-Sadr's chief representative in Sadr City, said the cleric had asked his offices in Baghdad and the southern city of Amarah to organize demonstrations demanding al-Maliki's resignation.

Nobody claimed responsibility for the attacks against the Green Zone, which houses the American and British embassies as well as the Iraqi government headquarters.

Bergner blamed Iranian-backed Shiite militia factions, saying most of the rockets had been fired from predominantly Shiite areas in eastern Baghdad, particularly Sadr City.

Several Iraqi civilians were killed or injured in separate fighting between U.S. troops and suspected al-Qaida in Iraq insurgents in the northern town of Tikrit, the U.S. military said.

The U.S. troops, who were targeting a militant suspected of organizing car bombs, called in an airstrike after coming under heavy fire from insurgents using civilians as shields when they approached a house, the military said.

At least seven bodies were found in the rubble of houses destroyed in the airstrike, an Iraqi police officer said on condition of anonymity because he was not authorized to release the information. He said a local judge who owned one of the houses was among the dead.