A roadside bomb killed three Shiite pilgrims Monday in the outskirts of Baghdad, while the death toll from a homicide bombing targeting pilgrims resting in a tent the day before rose to 56, authorities said.

In all, extremists have attacked pilgrims headed to the holy city of Karbala three times in the past two days.

The homicide bomber targeted travelers enjoying tea and refreshments in a tent near Iskandariyah, 30 miles south of Baghdad, as authorities have fortified the capital and Karbala to try to keep away extremists.

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Karbala is burial site of Imam Hussein, one of Shiite Islam's most revered figures, where ceremonies will culminate Wednesday to commemorate the end of the 40-day mourning period following the anniversary of his death.

Sunday's blast killed at least 56 people and injured 68, according to police and Dr. Mahmoud Abdul-Rida, director of the Babil health department.

Hours earlier, extremists attacked another group with guns and grenades in the predominantly Sunni Baghdad neighborhood of Dora, killing three and wounding 36, police said.

Monday's attack, meanwhile, also wounded 15, said a police official, who spoke on condition of anonymity because he wasn't authorized to release the information.

The U.S. embassy in Baghdad and U.S. military forces issued a joint statement Monday condemning the attacks.

"Those killed and wounded in yesterday's barbaric attacks in Baghdad and Iskandariyah were innocent citizens participating in an important religious commemoration," it said.

"This indiscriminate violence further reflects the nature of this enemy who will target even those practicing their religion in an effort to re-ignite sectarian strife in Iraq."

Major Shiite commemorations have frequently been targeted in the past by suspected Sunni insurgents led by Al Qaeda in Iraq in their drive to stoke sectarian violence.

The attacks have prompted U.S. and Iraqi forces to increase the number of checkpoints, and impose car bans and other measures in major Shiite cities to protect the worshippers.

Recent commemorations — including the Ashoura festival in mid-January to mark Imam Hussein's death — have passed without major bloodshed amid an overall decline in violence across Iraq.

But the pilgrims who walk for days to reach the shrine of Hussein, the grandson of the Prophet Muhammad who died in a seventh century battle near Karbala, are vulnerable despite the increased security.