The suicide bomber targeted travelers near Iskandariyah, as authorities have fortified the capital and Karbala to keep away extremists. Those are the main areas where rituals are performed to commemorate the end of the 40-day mourning period for Imam Hussein, one of Shiite Islam's most revered figures.
Major Shiite commemorations have frequently been targeted in the past by suspected Sunni insurgents led by al-Qaida 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.
The suicide bomber who struck on Sunday detonated his explosives belt at a tent providing food and drinks to the travelers on the southern outskirts of Iskandariyah, 30 miles south of Baghdad. The U.S. military and local authorities said the attack on a two-lane highway outside a residential area killed at least 40 people wounded 60 others.
Mohammad Mohsin Abbas, 35, said he was walking from Baghdad's main Shiite district of Sadr City and decided not to stop at the tent. He had just passed it when the blast occurred shortly before 3 p.m.
"Instantly I found myself knocked on the ground, unable to turn my head around to see what had happened," he said from his hospital bed after undergoing surgery to remove shrapnel in the nearby Shiite city of Hillah. "While they were transporting me to hospital, I saw that the tent had disappeared from the scene."
Abbas said he didn't believe anybody inside could have survived and the wounded must have been passers-by.
Iraqi police and U.S. troops at a nearby base away quickly responded, the military said. About 42,000 pilgrims had previously traveled through the same area without incident, the military added.
U.S. Col. Tom James, commander of the 4th Brigade, 3rd Infantry Division, which is responsible for the area, said Iraqi security forces had established extensive security along the pilgrimage routes as well as a cordon around Karbala.
"Unfortunately, just based on the complexity of these routes and the number of pilgrims it's difficult to completely secure them," he said in a telephone interview. "In this case, we think there was an attack from one of the flanks of these routes."
He said Iraqi troops quickly "re-established a safe environment for the rest of the pilgrims as they continued to walk south."
Suicide attacks and car bombings are frequently blamed on al-Qaida in Iraq, but James said it was too early to say who was behind Sunday's bombing, pending the investigation.
Iskandariyah was one of the main cities in an area dubbed "the triangle of death" for much of the U.S.-led war that began in March 2003. But it and other former insurgent strongholds have seen a steep drop in attacks in recent months that the U.S. military attributes to a Sunni revolt against al-Qaida in Iraq and an influx of American troops.
Sunni leaders denounced the bombing. Hard-line politician Adnan al-Dulaimi's bloc blamed the attack on foreigners "aiming to create sectarian strife and to destabilize the country."
Earlier, extremists killed three and wounded 36 in an attack on another group of pilgrims with guns and grenades in the Baghdad neighborhood of Dora, police said.
The assaults heightened tension around Arbaeen, when millions of pilgrims descend on Karbala, home to the golden domed mosques of Imam Hussein and his half brother Imam Abbas.
A steady stream of pilgrims - some carrying green, black or red banners - have been walking for days along the roads from Baghdad to the shrine. Among them were many families, including children and black-robed women.
In Karbala, crowds already choked the streets though the culmination of events is not until Wednesday.
Police Chief Raid Shakir Jawdat has said 40,000 police officers and military troops are being deployed to prevent attacks. At least 4 million pilgrims were already in the city, he said.
In other violence Sunday, two U.S. soldiers were killed in separate attacks in Baghdad, one in a roadside bombing and the other by small-arms fire, according to the military.
Separately, Prime Minister Nouri al-Maliki flew to London for his second round of medical tests in nearly two months.
Government spokesman Ali al-Dabbagh said al-Maliki, on his last visit over New Year's, underwent a cardiac catheterization, a routine procedure in which a thin tube is inserted into an artery or a vein to check for heart problems.
"This trip is only for a checkup," al-Dabbagh said. "There is nothing wrong with him. He was asked by the doctor to come back within six weeks, and that is why he is going."
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Associated Press writers Qassim Abdul-Zahra, Sinan Salaheddin and Bushra Juhi contributed to this report.
