Rockets and mortars pounded Baghdad's U.S.-protected Green Zone on Sunday and a suicide car bomber struck an Iraqi army post in the northern city of Mosul, two of several attacks nationwide that in all killed at least 78.

Men try to salvage their belongings inside their house in central Baghdad on Monday after it was hit by a rocket.
(Hadi Mizban/Associated Press)

The latest violence underscored the fragile security situation and the resilience of both Sunni and Shia extremist groups as the war enters its sixth year and the U.S. death toll hit 4,000 on Sunday.

Attacks in Baghdad probably stemmed from rising tensions between rival Shia groups — some of whom may have been behind the Green Zone blasts. It was the most sustained assault in months against the nerve centre of the U.S. mission.

The deadliest attack of the day was in Mosul, where a suicide driver slammed his vehicle through a security checkpoint in a hail of gunfire and detonated his explosives in front of an Iraqi headquarters building, killing 13 Iraqi soldiers and injuring 42 other people, police said.

Iraqi guards opened fire on the vehicle but couldn't stop it because the windshield had been bulletproofed, said an Iraqi army officer. He spoke on condition of anonymity because he was not supposed to release the information.

Mosul, Iraq's third largest city and about 360 kilometres northwest of Baghdad, has been described as the last major urban area where the Sunni extremist al-Qaeda group maintains a significant presence.

In Baghdad, rockets and mortars began slamming into the Green Zone about sunrise, and scattered attacks persisted throughout the day, sending plumes of smoke rising over the heavily guarded district in the heart of the capital.

A U.S. public address system in the Green Zone warned people to "duck and cover" and to stay away from windows.

At least five people were injured in the Green Zone, a U.S. Embassy statement said without specifying nationalities. The zone includes the U.S. and British embassies as well as major Iraqi government offices.

But Iraqi police said 10 civilians were killed and more than 20 were injured in rocket or mortar blasts in scattered areas of eastern Baghdad — some of them probably due to misfired rounds.

Also in the capital, seven people were killed and 14 wounded in a suicide car bombing Sunday in the Shia area of Shula, police reported. Such attacks are the hallmark of Sunni religious extremists.

Gunmen opened fire on passengers waiting for buses in a predominantly Shia area in southeastern Baghdad, killing at least seven men and wounding 16 people, including women and children, according to police.

Police also found the bullet-riddled bodies of 12 people — six in Baghdad, four in Mosul and two in Kut, scenes of clashes between government troops and Shia militiamen.

Elsewhere, several mortars or rockets struck a U.S. base in the Shia city of Hillah, about 96 kilometres south of Baghdad, Iraqi police said. The U.S. military did not immediately respond to a request for comment about the attack.

Shia extremists suspected in attacks

No group claimed responsibility for the Green Zone attacks, but suspicion fell on Shia extremists based in the areas from which the weapons were fired.

The attacks followed a series of clashes last week between U.S. and Iraqi forces and factions of the Mahdi Army, the biggest Shia militia loyal to radical cleric Muqtada al-Sadr.

Al-Sadr led two uprisings against U.S.-led coalition forces in 2004. Last August he declared a six-month ceasefire to purge the militia of criminal and dissident elements.

U.S. officials have cited the truce, which al-Sadr recently extended, among the reasons behind a 60 per cent drop in violence since President George W. Bush ordered 30,000 U.S. reinforcements to Iraq early last year.

But the ceasefire has come under severe strains in recent weeks. Al-Sadr's followers have accused the Shia-dominated government of exploiting the ceasefire to target the cleric's supporters in advance of provincial elections expected this fall.

Al-Sadr recently told his followers that although the truce remains in effect, they were free to defend themselves against attacks. Al-Sadr followers have demanded the release of supporters rounded up in recent weeks.

U.S. officials are targeting renegades

U.S. officials have insisted they are not going after Sadrists who respect the ceasefire but are targeting renegade elements, known as special groups, that the Americans believe have ties to Iran.

But the pattern of the attacks against the Green Zone could be a signal to the Americans and their Iraqi partners to ease their pressure against mainstream Sadrists or the special groups.

Elsewhere, 12 gunmen were killed Sunday in a raid against a suspected suicide bombing network east of Baqouba, the U.S. military said.

Iraqi police reported a dozen civilians killed in an air strike in the same area. But the military said those killed in the raid were insurgents, including six who had shaved their bodies, apparently in preparation for suicide operations.

A police commander was shot to death along with his driver in Balad Ruz, 72 kilometres northeast of Baghdad.

A roadside bomb near the northern city of Tuz Khormato killed four Iraqi soldiers, including an officer.

The violence was reported by police officials who declined to be identified because they weren't supposed to release the information.