>BAGHDAD (AP) – 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 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.