BAGHDAD - Prime Minister Nouri al-Maliki gave Shiite Muslim militiamen in Basrah three days to surrender as fighting raged Wednesday in the southern Shiite heartland and parts of Baghdad, leaving more than 80 people dead since the previous day.

Basrah residents trapped in their homes by raging gunbattles worried that food was running out with no end in sight to the clashes between Iraqi security forces, followers of radical Shiite cleric Muqtada al-Sadr and other armed factions.

Volleys of rocket and mortar fire shook the capital Wednesday, including the fortified Green Zone, which houses the U.S. Embassy and Iraqi government offices. One U.S. soldier, two American civilians and an Iraqi soldier were wounded in the attacks, the military said.

Two U.S. soldiers were killed by hostile fire in separate attacks Wednesday in Baghdad, the military said without naming the assailants. The deaths lifted to at least 4,002 the number of American personnel killed since the start of the U.S.-led invasion of Iraq in March 2003, according to the independent Web site icasualties.org.

Fighting erupted Tuesday in Basrah when Iraqi government security forces announced the launch of a crackdown against armed factions and criminal gangs that are vying for control of the city and its lucrative oil industry. By Wednesday, more than 30 people had been killed and 100 injured in Iraq's second- largest city, health officials said.

The level of resistance represented a major challenge to al-Maliki's authority and deepened fears that a cease-fire declared last year by al-Sadr might be in danger of collapse. The truce has played a key part in the significant drop in violence since a U.S. troop buildup reached its peak in June.

Al-Sadr's followers have charged for months that American and Iraqi security forces, many of them with ties to rival Shiite factions within the government, are taking advantage of the truce to arrest members of al-Sadr's Mahdi Army militia and weaken his movement ahead of provincial elections slated for Oct. 1. Al-Sadr's representatives called Tuesday for nationwide protests in response to the latest crackdown.

Al-Maliki, who is overseeing the campaign from Basrah, gave gunmen 72 hours to turn themselves in, surrender their weapons and sign a written pledge renouncing violence or face what he termed serious penalties.

An offer of talks

Al-Sadr himself issued no comment. But officials at his Najaf headquarters said the cleric was urging his followers to respect the truce and would send representatives to Basrah to negotiate with local leaders.

At the same time, Liwa Sumeysim, who heads al-Sadr's political operations nationwide, said al-Maliki's presence and the deployment of 3,500 extra police and soldiers to Basrah were a provocation and they should leave.

Members of al-Maliki's governing coalition maintained Wednesday that the crackdown was not aimed at al-Sadr but at the "outlaws" and "criminals" who have infiltrated Basrah's government, security forces and oil industry through violence and intimidation.

U.S. officials in Washington voiced approval Wednesday for the offensive.

Yet the spike in violence, particularly in Baghdad's Sadr City neighborhood, could compromise administration claims that an 18-month troop buildup has led to greater stability and safety across Iraq.

U.S. officials have credited al-Sadr's cease-fire for helping bring down violence Iraq. But few American officials voiced concern Wednesday that the aggressive measures in Basrah would jeopardize the cease-fire.

Steven Hadley, President Bush's national security adviser, said the offensive is aimed at militia members who have continued to fight in spite of al-Sadr's order to lay down arms.

"It's not a move, as we read it, by the central government to repudiate the cease-fire in any way," Hadley said.