BAGHDAD (AP) _ 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 attacks on the Green Zone probably stemmed from rising tensions between rival Shiite groups and were the most sustained assault in months against the nerve center of the U.S. mission.