BAGHDAD (Reuters) -- A female suicide bomber infiltrated a crowd of Shi'ite pilgrims, killing at least 35 people and wounding at least 79 at a Shi'ite shrine in Baghdad.
The blast struck a checkpoint outside the Imam Musa al-Kadhim shrine in Al-Kadhimiyah, a mainly Shi'ite area of Baghdad, as Shi'a prepared for the Ashura holiday this week to mark the death of Hussein, the grandson of the Prophet Muhammad.
Many of the casualties were pilgrims from Iran, security spokesman Major General Qasim al-Musawi said, underscoring the deep religious ties between the two majority Shi'ite countries.
"A woman wearing an explosive vest managed to reach one of the security checkpoints near the revered Kadhim shrine and exploded herself among a crowd of pilgrims," his statement said.
He gave an initial toll of 35 killed and 79 wounded. Iraqi security sources gave slightly higher casualty totals.
U.S. forces in Iraq came under an Iraqi mandate on January 1 in step with a bilateral pact that will require the withdrawal of the 140,000 U.S. troops by the end of 2011.
As the United States reduces its activities in Iraq, local forces are taking greater responsibility for security.
The bomb attack was a reminder of the challenges they face, almost six years after the U.S.-led invasion in 2003.
Violence has dropped dramatically from the peak of sectarian bloodshed in 2006-07, but militants regularly stage bombings.
Al-Musawi said Prime Minister Nuri al-Maliki had ordered a committee to investigate the attack and punish those behind it.
"We are taking a series of measures to prevent a repeat of this security violation," he told Reuters, adding that security forces had been beefed up all around Al-Kadhimiyah.
Hundreds of thousands of Iraqi Shi'a will visit the holy city of Karbala, 80 kilometers southwest of Baghdad throughout the Ashura week to mark the death of Hussein, one of the most important events in the Shi'ite calendar.
Sunni militants have frequently targeted Shi'ite pilgrimages, which have become massive events since the fall of Saddam Hussein, who repressed them.
In 2004, at the first Ashura pilgrimage after Saddam's fall, Sunni militants killed more than 160 Shi'ite pilgrims in coordinated attacks on the Al-Kadhimiyah and Karbala shrines.
Those strikes -- at that point the bloodiest day since Saddam's fall -- were an early portent of the sectarian fighting that would consume Iraq over the next few years.
But despite the violence, pilgrimages continue to attract hundreds of thousands of worshippers, many from Iran.
Sunni militants use women and girls as suicide bombers to get them past tightened security. At least two dozen female bombers struck last year, killing scores of people.
U.S. forces are slowly disengaging from day-to-day patrols as they prepare to withdraw forces from towns by mid-2009.
On January 5, U.S. forces put the Iraqi government in charge of mainly Sunni Arab tribal guards in Diyala Governorate, north of Baghdad.
The guards will now be paid by the Iraqi government, which says it will hire 20 percent of them into the police and army, and gradually give the rest civilian jobs or training.