The violence followed a vow by al-Qaeda's new leader in Iraq to avenge the death of his predecessor Abu Musab al-Zarqawi, who was killed in a US air strike on June 7.

In the deadliest attack, a car bomb targeting Iraqi army and police killed 11 people.

Reuters Television footage showed the blackened remains of at least six burnt-out cars. A charred body was taken on a stretcher to an ambulance. A man with blood on his face stood nearby looking stunned and smoking a cigarette.

US military helicopters and divers searched for two US soldiers missing after an attack on Friday in which one American soldier was killed in the insurgent bastion of Yusufiya in the "Triangle of Death" south of Baghdad.

"We are using all available assets, Coalition and Iraqi -- ground, air and water -- to locate and determine the duty status of our soldiers," Major General William Caldwell, the spokesman for the US military in Iraq, said in a statement.

He said teams of divers were working the canals and Euphrates river near Yusufiya, a rural area which has seen fierce fighting between US forces and al Qaeda militants.

More than 2,500 US soldiers have died in Iraq since the US-led invasion in 2003.

Prime Minister Nuri al-Maliki, under pressure to rein in violence that has killed tens of thousands of Iraqis, launched a security sweep on Wednesday with 50,000 Iraqi troops backed by 7,000 US troops to pile pressure on al-qaeda.

But the operation, mounted one day after US President George W. Bush made a surprise visit to Baghdad to bolster Maliki's month-old government, has failed to stop attacks.

In more violence of the type carried out by al-qaeda, a bomb killed six people and wounded 11 in a crowded market in central Baghdad and mortar rounds killed two people and wounded 14 in another market in the Shi'ite district of Kadhimiya.

Attacks on crowded markets are a common tactic used by al-qaeda as part of what US officials say is a campaign to ignite a sectarian civil war between majority Shi'ites and Saddam Hussein's once-dominant Sunni Arab minority.

In the town of Mahmudiya just south of the capital, a car bomb targeting an Iraqi army checkpoint killed seven people.

Four days into the crackdown, the Interior Ministry has not yet announced any arrests or other results.

As the wounded from Saturday's blasts were being treated, state television broadcast footage of Iraqi soldiers marching to the sound of martial music, part of a government campaign to bolster the image of its security forces.

The US military has said it expects al-qaeda's new leader, who it identified as Abu Ayyub al-Masri, to use the same tactics as Zarqawi, a Sunni Arab militant who had concentrated mass attacks against Shias.