As many as 18 were killed in a double attack by two women suicide bombers who blew themselves up among a crowd of pilgrims heading to the city, police Lieutenant Kazem al-Khafaji in Babil province said.

The women detonated their explosives-packed vests 50 metres apart and at a five-minute interval in Iskandiriyah 60 kilometres south of the capital, Lt Khafaji added.

The twin suicide attack left at least 75 more wounded, most of them young men, in the most lethal attack to hit the war-torn nation since last Friday when 21 people were killed in Tal Afar by a car bomb.

Mr Haider Kadhum, 20, a pilgrim from Baghdad who was being treated for wounds to his feet and back at Hilla hospital, said he had been drinking water among a group of pilgrims when he felt the blast.

'In front of us was a large group of pilgrims and suddenly we felt strong hot waves of fire with the bang of the explosion and people's bodies were ripped apart', he told AFP.

Earlier on Thursday another Shi'ite pilgrim was killed and seven others were wounded by a roadside bomb in Baghdad's commercial district of Karrada as they set off for Karbala, around 110 kilometres south of Baghdad, for Sunday's festival.

Another explosion killed a policeman and wounded five of his colleagues near a checkpoint in the Zafraniya district of southern Baghdad set up to search pilgrims heading south.

Although the incidence of bombings has dropped to four-year lows, the latest wave of violence highlighted the difficulty Iraqi forces face in maintaining an uncertain peace, especially during large public gatherings.

Bloodshed routinely marks Shi'ite pilgrimages, the last of which was on July 28, when three female suicide bombers killed 25 Shi'ites near a shrine in Baghdad.

This weekend tens of thousands of Shi'ites are expected to flock to Karbala to venerate Imam Mahdi, an eighth century imam who vanished as a boy and whom Shi'ites believe will return to bring justice to the world.

The Shi'ite community was once led by a series of infallible imams who were direct descendants of the Prophet Mohammed and his son-in-law Ali. When the Mahdi went into hiding, leadership of the community passed to the clergy.

In other violence, a car bomb targeting a police patrol near the restive city of Baquba, about 60 kilometres north of Baghdad, killed two policemen and wounded six, security officials said.

The defence ministry said that on Tuesday the Iraqi army discovered dozens of houses that had been booby-trapped with explosives by Al-Qaeda jihadists in the same area - about 10 kilometres southeast of Baquba.

Also near Baquba, a bomb hidden in a field killed a 10-year-old girl, a security official said.

Meanwhile, 370 kilometres north of Baghdad, in the Al-Qaeda bastion of Mosul, three Iraqi soldiers were killed, one by a roadside bomb and two from sniper fire, police sources said.

A suicide bomber also killed one policeman 60 kilometres north of Mosul in the ancient city of Nimrud. -- AFP