Rockets and mortars pounded Baghdad's US-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.

A US soldier stands guard in the area where a suicide car bomber detonated his load in Shula, Baghdad, Iraq, Sunday, March 23, 2008. [Agencies]

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 US 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 US mission.