A suicide bomber set off two explosions in a crowd of Shia pilgrims heading to a holy city south of Baghdad on Friday, killing at least 40 and wounding about 150 people, Iraqi officials said.

The attack was the third in the last week against Shia pilgrims making their way to the city of Karbala to mark the holy day of Arbaeen, which marks the end of 40 days of mourning after the anniversary of the death of Imam Hussein, a revered Shia figure.

There was no immediate claim of responsibility for Friday's blasts, but Nouri al-Maliki, the Iraqi prime minister, blamed al-Qaeda and a group loyal to former Iraqi president Saddam Hussein.

Two days earlier, a motorcycle bomb exploded, killing dozens. On Monday, a female suicide bomber killed at least 54 pilgrims in an attack just north of Baghdad.

Friday's attack began shortly after noon when a parked car exploded just east of one of the main entrances to Karbala, two Health Ministry officials said. The explosion sent pilgrims running down the highway and into the path of a suicide car bomber who detonated the vehicle, they said.

An Iraqi police official gave a conflicting account, however, saying two mortar rounds struck the area, followed by the suicide car bomb.

The crowds trying to enter the city made it difficult for ambulances to get to the wounded, another police official said. The officials spoke on condition of anonymity because they were not authorized to release the information.

The concentration of Shia Muslims in Karbala has made the annual ceremony a prime target for Sunni militants.

The sectarian violence in Iraq comes as Iraq's Shia-led government and Sunni politicians argue over a push to ban Sunni candidates with alleged ties to Saddam's regime from next month's election.

A parliamentary committee blacklisted more than 450 politicians allegedly loyal to Saddam, but an Appeals Court overturned the ban on Wednesday.

With files from The Associated Press