Insurgents carried out a series of deadly assaults in Iraq on Friday, killing at least 40 people as United States forces claimed a heavy blow to Iraq's most wanted man, Abu Musad al-Zarqawi.

In the deadliest attack on Friday, a suicide car bomb detonated at a crowded market in Suwayrah, some 50 km south of Baghdad, killing at least 22 people and injuring more than 40 others.

The mostly Shi'ite city lies in an area where Sunni militants have staged dozens of assaults on the fledging Iraqi security forces, reinforcing fears such attacks may lead to a civil war.

The deadly attack came after another suicide bomber blew up a booby-trapped car at a checkpoint in the northern city of Tikrit, the hometown of former Iraqi ruler Saddam Hussein, killing seven policemen and wounding several others.

"A suicide bomber drove an explosive-laden vehicle into a police bus near a checkpoint manned by Iraqi army and policemen on a bridge in Tikrit," Col. Hassan Ahmed told Xinhua.

In the northern city of Mosul, a car bomb hit a police patrol, killing four police commandos and five passers-by, police said.

The latest insurgent attacks were part of at least 18 suicide bombings that have caused about 500 casualties since the transitional government was approved by the parliament a week ago, casting doubts over the new government's ability to calm down the instability.

Also on Friday, Iraqi police said that they found 14 bodies who had been shot dead and buried in northeastern Baghdad.

Some of the bodies, which were believed to be Iraqis, were blindfolded and had been shot in the head, the police said.

A US military spokesman confirmed the incident, but did not give further information.

Many dead bodies had been found in several areas in Iraq, mostly for police and security forces, as disturbance and violence surged in the country after the US-led invasion in 2003.

Iraqi officials often blame such attacks on the elusive al Qaeda leader in Iraq, Jordanian militant Abu Musab al-Zarqawi, whose group has claimed responsibility for Friday's Tikrit bombing.

"A lion from the martyrs' brigade attacked a group of the apostate police, who are agents of America, in the city of Tikrit ... killing many," said a statement from Al Qaeda Organization for Holy War in Iraq on an Islamist Web site.

However, the US military said it had captured or killed more than 20 top lieutenants and other senior members to Zarqawi in recent months, an apparent heavy blow to the notorious militant at large. 

Iraqi Prime Minister Ibrahim al-Jaafari's office also said Friday the security forces had collected significant information on Zarqawi's terror network in Iraq through the confessions of his driver.

In a statement, the office said that Zarqawi's driver shed light on the weakness of the terror network as a result of the capture and killing of many leaders of the terror groups in Iraq by the security forces.

The driver confessed that the Iraqi security forces were about to capture Zarqawi near the Euphrates River in the area between Haditha and Hit on Feb. 20, the statement said.

Zarqawi, with a bounty of 25 million dollars on his head, had managed to escape whereas his driver was seized along with Zarqawi's personal computer.

Further information provided by the driver might lead to the capture of other key elements in the network, the statement said, adding he also disclosed the external resources for the terrorist groups.

In other developments, kidnappers of an Australian hostage has set a 72-hour ultimatum for Australia to start withdrawing its troops from Iraq, the Qatar-based al-Jazeera TV channel reported Friday.

The TV news channel, showing footage of Australian Douglas Wood, said "the group calling itself the Shura Council of the Mujahedeen of Iraq, which is holding Australian hostage Douglas Wood, gave Australian authorities 72 hours to start withdrawing their forces from Iraq."

The TV station did not specify what the militants would do if their deadline isn't met, but a number of previous hostages have been killed.

Earlier in the day, the Australian authorities said a task force that rushed to Baghdad this week had established Wood is still alive.

Meanwhile, The TV news channel reported that six Jordanians working in Iraq have been kidnapped by a militant group named itself as al-Bara bin Malik Brigades.

The six hostages, shown seated on the floor holding their passports, were said to have been working with the US-led forces in Iraq. The militant group warned Jordanian companies against working with US forces.

More than 200 foreigners have been kidnapped in Iraq since Saddam Hussein's regime collapsed in April 2003. Some of the kidnappers have sought ransom, while others pursued political motives such as the withdrawal of foreign companies and troops from Iraq.

(Xinhua News Agency May 7, 2005)