BAGHDAD — A roadside bomb killed four U.S. soldiers in Baghdad on Sunday, the military said, pushing the overall American death toll in the five-year war to at least 4,000. The grim milestone came on a day when at least 61 people were killed across the country.

Rockets and mortars pounded the U.S.-protected Green Zone, underscoring the fragile security situation and the resilience of both Sunni and Shiite extremist groups despite an overall lull in violence.

The soldiers with Multi-National Division _ Baghdad were on a patrol when their vehicle was struck at about 10 p.m. in southern Baghdad, the military said. Another soldier was wounded in the attack.

Identities of those killed were withheld pending notification of relatives.

Navy Lt. Patrick Evans, a military spokesman, expressed condolences to all the families who have lost a loved one in Iraq, saying each death is "equally tragic."

"There have been some significant gains. However, this enemy is resilient and will not give up, nor will we," he said. "There's still a lot of work to be done."

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.

A U.S. official, speaking on condition of anonymity because the official was not authorized to release the information, said those injured included an American and four third-country nationals, meaning they were not American, British or Iraqi.

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.

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.

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

Last year, the U.S. military deaths spiked as U.S. troops sought to regain control of Baghdad and surrounding areas.

The death toll has seesawed since, with 2007 ending as the deadliest year for American troops at 901 deaths. That was 51 more deaths than 2004, the second deadliest year for U.S. soldiers.

The 4,000 figure is according to an Associated Press count that includes eight civilians who worked for the Department of Defense.

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Associated Press Writer Kim Gamel contributed to this report.