A homicide car bomber detonated explosives in a crowd of laborers gathered across the street from a major Shiite shrine in southern Iraq Tuesday, killing at least 53 people and wounding 105, officials and witnesses said.

The attacker drove a minivan to where Shiite laborers gather daily to look for work in Kufa, 100 miles south of Baghdad. He offered them jobs, loaded the minivan with volunteers and then detonated the vehicle, Najaf Gov. Asaad Abu Kalal told a Shiite television station.

CountryWatch: Iraq

The blast occurred about 7:30 a.m. across the street from Kufa's gold-domed mosque, police Capt. Nafie Mohammed said. The shrine, located in a congested area of the city, marks the place where Imam Ali, cousin and son-in-law of the Prophet Muhammad, was mortally wounded.

Senior provincial health official Dr. Muthir al-Ithari said the casualty figure had been consolidated from reports sent by hospitals in Kufa and nearby Najaf.

Prime Minister Nouri al-Maliki, a Shiite, condemned the attack and promised to track down and punish those who planned it.

Kufa is a stronghold of radical anti-American cleric Muqtada al-Sadr, whose movement controls the mosque. It appeared the blast was aimed at undermining al-Sadr's position in Iraq's sectarian struggle, much of which has been blamed on al-Sadr's Mahdi Army militia.

The blast occurred one day after gunmen killed at least 50 people in a major assault on a market in Mahmoudiya, 75 miles north of Kufa. Witnesses said the assault began with an attack on mourners at a funeral for one of al-Sadr's militiamen.

Women and children were among the dead and wounded in that assault, hospital officials said. Late Monday, police said they found 12 bodies in different parts of town — possible victims of reprisal killings.

Several witnesses, including municipal council members, said the attack began when gunmen — presumed to be Sunnis — fired on the Mahdi Army funeral, killing nine mourners.

Assailants then drove to the nearby market area in the town 20 miles south of Baghdad, killing three soldiers at a checkpoint and firing rocket-propelled grenades and automatic rifles at the crowd.

The assault occurred a few hundred yards from Iraqi army and police positions, but the troops did not intervene until the attackers were fleeing, the witnesses said. They spoke on condition of anonymity because of fear of reprisals.

The U.S. command announced that three American soldiers were killed in separate attacks Monday — two in the Baghdad area and one in Anbar province west of the capital. At least 2,554 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

There were conflicting casualty figures in the market attack, with a Shiite television station reporting more than 70 dead. But local police and Dr. Dawoud al-Taie, director of Mahmoudiya hospital, said 50 people were killed and about 90 were wounded.

In the first 17 days of July, at least 617 Iraqis have been killed in war-related violence, at least 527 civilians and 90 police and security forces, according to an AP count. In the nearly two months since the unity government took office on May 20, more than 1,850 Iraqis have been killed, including at least 1,585 civilians and 267 security forces. The figures do not include insurgents.

The July figure represents a marked increase over the same period last year when an AP count showed at least 450 Iraqis killed, at least 306 civilians and 144 police and security forces. The 617 killed so far this July, not including Tuesday's homicide attack, is already near the total killed in all July last year: 714.

The Shiite television station Al-Forat broadcast strident quotes from Shiites who blamed the attack on Sunni religious extremists. They expressed outrage that Sunni politicians could not rein in the militants.

The main Sunni bloc in parliament said the attack may have been retaliation for the kidnapping of seven Sunnis whose bodies were found Sunday in Mahmoudiya. The bloc accused Shiite-dominated Iraqi security forces of failing to control the situation.

The events also raised doubts about the effectiveness of the U.S. strategy of handing over large areas of the country to Iraqi control, while keeping U.S. troops in reserve.

U.S. troops of the 101st Airborne Division reported hearing detonations and gunfire, the U.S. command said. But Iraqi troops are responsible for security in Mahmoudiya, and American soldiers do not intervene unless asked by the Iraqis.

Four soldiers and a former soldier from the division are accused of raping and murdering a teenage girl near Mahmoudiya on March 12. A sixth soldier is accused of failing to report the crime.

The Mahmoudiya attack was part of a rising tide of tit-for-tat killings and intimidation that many Iraqis fear is the prelude to civil war. The campaign of intimidation and attacks is slowly transforming Baghdad into sectarian zones under the tacit control of armed groups that protect members of their sect.