The Iranian pilgrims, meanwhile, were driving through the Sunni neighborhood of Dora in Baghdad early Wednesday when their bus was hit by a roadside bomb, officials said. The Iranians were traveling to Baghdad from the holy city of Karbala, the officials added, speaking on condition of anonymity because they were not authorized to talk to the media.
Hundreds of thousands of Shiites are expected to visit Karbala and other shrines across Iraq in the coming days for the 10-day festival known as Ashoura. The event marks the anniversary of the 7th century death of Imam Hussein, who was slain in battle near Karbala.
Hussein's death led to the Sunni-Shiite split -- the centuries-old divide that provided the backdrop for the sectarian bloodshed in Iraq after the 2003 U.S.-led war. The invasion toppled the Sunni-dominated regime of Saddam Hussein, which had long persecuted the country's Shiite majority.
Sunni insurgents in Iraq have frequently targeted Shiite shrines and large religious festivals, and killed thousands of pilgrims since 2004. They have also attacked Iranian pilgrims, very few of whom were allowed to visit Iraq's holy sites under Saddam.
Wednesday's attack comes four days after bombings near a Shiite shrine in Baghdad killed eight Iranian pilgrims.
Ashoura is expected to peak at the end of next week when the pilgrims -- many walking hundreds of miles to reach Karbala to commemorate Imam Hussein's suffering while dying on the battlefield -- converge on the city.
Authorities have beefed up security around Karbala, including imposing a vehicle ban that forces pilgrims to enter the city on foot. Additional checkpoints have been erected at several entrance points and 25,000 police officers have been deployed to search the pilgrims at various points leading up to the mosque where Imam Hussein is buried.