At least 100 Shiite pilgrims were killed and more than 200 wounded when they were heading for Iraq's holy Shiite city of Karbala on Tuesday, underscoring the relentless violence despite a recent massive security crackdown.

Two suicide bombers blew themselves up among a crowd of Shiite pilgrims in Hilla, some 100 km south of the capital, leaving 90 people dead and 160 others injured, a local police source told Xinhua by telephone.

The attack occurred when hundreds of Shiite pilgrims were gathering on the main street of the city on their way to Karbalato commemorate Arba'een (40 days after the day of Ashura), which marks the death of Imam al-Hussein bin Ali, the grandson of Prophet Mohammed, who was killed in the seventh century.

The Shiite-dominated city of Hilla has been repeatedly hit by bombings in the past. On Feb. 28, 2005, a car bomb attack at a medical center killed more than 110 civilians and wounded around200.

Militants also carried out attacks against Shiite pilgrims in the capital. At least 10 Shiite pilgrims were killed and 42 others injured in three car bomb attacks in Baghdad, according to an Interior Ministry source.

Some analysts fear that the latest upsurge of attacks targeting Shiite pilgrims could fuel sectarian tension between the Shiite majority and Sunni minority and cast shadows on the ongoing security plan, which is widely seen as the last chance to restore peace and stability in the war-torn country.

Meanwhile, the U.S. military suffered a heavy loss with nine of its servicemen killed and four injured in two bomb attacks north of Baghdad on Monday and Tuesday.

Three Task Force Lightning soldiers were killed and another injured as a result of injuries sustained following an explosion near their vehicles, the military said in a statement on Tuesday.

The incident took place when the soldiers were conducting combat operations in Diyala province northeast of Baghdad on Monday, the statement said.

An earlier statement said that six U.S. soldiers were killed and three others wounded when their vehicles were hit by a roadside bomb in Salahud in province north of the capital on Tuesday.

The soldiers, assigned to the same unit, were conducting combat operations in the volatile predominantly Sunni province.

About 3,180 U.S. soldiers have been killed in Iraq since the U.S.-led war on Iraq broke out in March 2003, according to media count based on Pentagon figures.

Editor: Yan