BAGHDAD, Iraq (CNN) -- Shiite Muslim cleric Muqtada al-Sadr urged the Iraqi government to end a crackdown on his militia after two days of fighting left more than 100 people dead across southern Iraq, an aide said Wednesday.
Iraqi Mehdi Army fighters take position during clashes in the southern city of Bara.
Heavy fighting between government troops and members of al-Sadr's Mehdi Army continued late Wednesday in the southern city of Basra, Iraq's second-largest city and a major oil port, a provincial official said.
Prime Minister Nuri al-Maliki is leading the operation personally, his office said, with U.S. and British troops playing support roles.
The fighting threatens to unravel a seven-month cease-fire with al-Sadr's supporters. U.S. commanders credit it with tamping down Iraq's sectarian warfare.
The government said it is targeting only "outlaws" who have ignored al-Sadr's cease-fire order, first issued in August and renewed in February.
The prime minister has given militants a 72-hour ultimatum to surrender, but al-Sadr's followers say they have been unfairly targeted by Iraqi and U.S. raids.
The worst casualties were reported in Basra, where between 40 and 50 people had been killed by late Wednesday. In Kut, where al-Sadr's support runs strong, police said 35 people were dead; in Baghdad, 22; and in Diwayniya, one. Watch a gun battle in Basra »
Sheikh Salah al-Obaidi, an aide to al-Sadr in Najaf, said the cleric has called on al-Maliki to leave Basra and let tribal leaders and political parties resolve the security problems through peaceful dialogue. Al-Sadr's political party quit Iraq's government last year after al-Maliki refused to demand a timeline for the withdrawal of American forces from Iraq.
But an official with Basra's provincial council, who spoke on condition of anonymity, said al-Maliki told provincial officials he would remain in the city even if the battle lasted a month.
Al-Sadr has called for a campaign of civil disobedience by his supporters, but has not rescinded his cease-fire order.
Since the fighting began on Tuesday, clashes between government troops and militiamen have spread to Baghdad and the southern provincial capitals of Kut, Hilla and Diwaniaya and the Shiite holy city of Karbala, Iraqi authorities reported.
The International Committee of the Red Cross has announced it is providing supplies to hospitals in Basra and Baghdad's Sadr City district to help treat several hundred people it said had been wounded.
Meanwhile, two American soldiers and one Briton were reported dead in firefights Wednesday, and another American died of wounds suffered in the shelling of Baghdad's International Zone earlier this week, U.S. and British authorities said. The zone, which houses the U.S. Embassy and Iraqi government offices, was hit by rocket attacks four times Wednesday, the U.S. military reported.
Tensions between the Sadrist movement and the Iraqi government, which is dominated by rival Shiite parties, have been growing for months. The cleric's followers have fought street battles in southern Iraqi cities such as Diwaniya and Kut with security forces linked to the leading Shiite party in Iraq, the Islamic Supreme Council of Iraq, and its Badr Brigade militia.
U.S. commanders have blamed the fighting on "rogue" or "outlaw" elements of al-Sadr's militia and Shiite "special groups" who receive support from Iran -- allegations Iranian officials deny.
The Basra operation came after months of jockeying for power among the Sadrists, the Fadhila party and the ISCI -- a situation CNN military analyst Don Shepperd compared to the gang wars that wracked Chicago, Illinois, in the 1920s. Al-Maliki's move is an attempt to reassert the central government's control over the city, but Shepperd warned the operation is likely to result in "significant combat in a very highly populated area."
"This is going to be ugly for the people of Basra, and it's going to be a black eye against the central government," said Shepperd, a retired U.S. Air Force major general.
He said public confidence in central government authority "is the key to the future of Iraq," but the prime minister is taking a big risk by moving against the militias.
"This can break down in so many ways," he said. "It can affect security across Iraq. It can affect the confidence of the people in the central government, which the U.S. is trying to build so it can get out. So this is a very complex thing, and it can go wrong for the U.S. in many ways."
The Basra provincial official said at least five Basra neighborhoods are under Mehdi Army control. Local leaders have warned al-Maliki that if he fails to restore order, the city would fall into the hands of outlaws.
The operation has drawn praise from U.S. officials, however. In Washington, Pentagon spokesman Geoff Morrell told reporters the offensive was "a sign that the Iraqi security forces are now capable of confronting, fundamentally, their problems."
And at the White House, national security adviser Stephen Hadley told reporters the Iraqi government is cracking down on groups "that have stepped outside of the rule of law" and said it should not imperil the cease-fire.
"The consequence is, the militias and criminal groups have reacted with violence," Hadley said. "But it is in reaction to an effort by governmental authorities to reassert control in an area which had become pretty lawless. And so, in some sense, this is an indication of the continued maturation of this government in its willingness and capacity to take increasing responsibility for security." E-mail to a friend
CNN's Mohammed Tawfeeq and Thomas Evans contributed to this report.