Baghdad ? Iraq’s prime minister warned gunmen in the oil port of Basra to surrender their weapons by Friday or face harsher measures, as clashes between security forces and Shiite militia fighters spread throughout the south and in Baghdad.

Despite Prime Minister Nouri al-Maliki’s ultimatum Wednesday, government troops in Basra were having trouble making inroads into neighborhoods that the radical Shiite cleric Muqtada al-Sadr’s Mahdi Army has controlled for years.

Residents spoke of militiamen using mortar shells, sniper fire, roadside bombs and rocket-propelled grenades to fight off security forces.

A Pentagon official said reports from the Basra area indicate that militiamen had overrun a number of police stations and that it was unclear how well the Iraqi security forces were performing overall. The official spoke on condition of anonymity because he was not authorized to discuss the matter publicly.

Al-Maliki, a Shiite, remained in Basra to supervise a crackdown against the spiraling violence between militia factions vying for control of the center of Iraq’s vast oil industry, located near the Iranian border. The events threatened to unravel a Mahdi Army cease-fire and spark a dramatic escalation in violence after a monthslong period of relative calm.

Street battles that broke out Tuesday in Basra and Baghdad’s main Shiite district of Sadr City spread to several other neighborhoods and southern cities, leaving nearly 140 dead, including civilians, Iraqi security forces and militants. That two-day figure was a rough estimate provided by police and hospital officials who could not give a more specific breakdown.

In Baghdad, 16 rockets slammed into the U.S.-protected Green Zone, the U.S. military said, as the heavily fortified area was hammered for the third time this week. One soldier with the U.S.-led coalition, two American civilians and an Iraqi soldier were wounded in the attacks, it said.

At least 11 Iraqis were killed elsewhere in the capital by rounds that apparently fell short, police said.

Two American soldiers were also killed Wednesday in separate attacks in Baghdad, the military said, raising the overall U.S. death toll since the war started more than five years ago to at least 4,003, according to an Associated Press count.

The Sadrists are angry over recent raids and detentions, saying U.S. and Iraqi forces have taken advantage of the August 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 campaign after the Iraqi parliament agreed in February to hold provincial elections by the fall.

The U.S. military insisted the fight was not against al-Sadr’s movement but breakaway factions believed to be funded and trained by Iran, which has denied the allegations.