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.