BAGHDAD - Rockets and mortars pounded Baghdad's U.S.-protected Green Zone Sunday and a suicide car bomber struck an Iraqi army post in the northern city of Mosul in a surge of attacks that killed at least 57 people nationwide.
The latest violence underscored the fragile security situation and the resilience of both Sunni and Shiite extremist groups as the war enters its sixth year and the U.S. death toll in the conflict approaches 4,000.
Attacks in Baghdad probably stemmed from rising tensions between rival Shiite groups - some of whom may have been behind the Green Zone blasts. It was the most sustained assault in months against the nerve center of the U.S. mission.
The deadliest attack of the day was in Mosul when 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 about 225 miles northwest of Baghdad, has been described as the last major urban area where the Sunni extremist al-Qaida 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 Shiite area of Shula in the capital, police reported. Such attacks are the hallmark of Sunni religious extremists.
Gunmen opened fire on passengers waiting for buses in a predominantly Shiite 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, scene of clashes between government troops and Shiite militiamen.
Elsewhere, several mortars or rockets struck a U.S. base in the Shiite city of Hillah, about 60 miles south of Baghdad, Iraqi police said. The American military did not immediately respond to a request for comment about the attack.
No group claimed responsibility for the Green Zone attacks, but suspicion fell on Shiite extremists based on 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 Shiite 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 cease-fire 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 percent drop in violence since President Bush ordered 30,000 U.S. reinforcements to Iraq early last year.
But the cease-fire has come under severe strains in recent weeks. Al-Sadr's followers have accused the Shiite-dominated government of exploiting the cease-fire 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 have insisted they are not going after Sadrists who respect the cease-fire 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 airstrike 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, 45 miles northeast of Baghdad.
A roadside bomb near the northern city of Tuz Khormato killed four Iraqi soldiers, including an officer.