A female suicide bomber killed 40 people in a tent filled with women and children resting during a pilgrimage to a Shia holy city on Friday, Iraqi officials said.

Sixty others were injured in the deadliest attack in Iraq this year, on the third straight day of bombings against Shia pilgrims.

A police official said the bomber detonated her explosives among pilgrims walking to the holy city of Karbala, south of Baghdad, for Shia religious ceremonies. He spoke on condition of anonymity because he was not authorized to talk to the media.

Interior Ministry spokesman Maj.-Gen. Abdul-Karim Khalaf confirmed the attack between the cities of Mussayib and Iskandariyah about 60 kilometres south of Baghdad.

Most of the dead and wounded in Friday's attack were women and children, said Mohammed Abbas, a medical official in Mussayib.

The attacks against the pilgrims appear to be part of a Sunni extremist campaign to rekindle the sectarian conflict that nearly plunged the country into full-scale civil war two years ago.

On Thursday, a suicide bomber detonated an explosive belt packed with nails among Shia worshippers in Karbala near the revered Imam Hussein shrine, killing eight pilgrims and wounding more than 50.

A day earlier, at least 12 people were killed and more than 40 wounded in bombings in Baghdad that targeted Shia pilgrims traveling to Karbala, 80 kilometres to the south.

Iraqi officials have mounted an extensive security operation to protect the pilgrims, who will be celebrating Monday's end of 40 days of mourning that follow Ashoura, the anniversary of the seventh-century death of the Prophet Muhammad's grandson Hussein.

He was killed in a battle near Karbala for the leadership of the nascent Muslim nation following Muhammad's death in 632. His death led to the split between Sunni and Shia Muslims.

About 40,000 Iraqi troops have been deployed along major routes to Karbala, and officials say security cameras have been installed near the Imam Hussein shrine to keep a lookout for possible threats.

Despite strict security, al-Qaeda and other extremist groups have frequently targeted Shia pilgrims during religious commemorations, which were severely curtailed under Saddam Hussein's Sunni-dominated regime.

Last March, a female suicide bomber attacked Shia worshippers in Karbala, killing at least 49. At least 85 people died in a suicide bombing in Karbala in March 2004.

The chief United Nations official in Iraq, Staffan de Mistura, has said the attacks against pilgrims were "clearly designed to sectarian tensions" that many Iraqis hope are behind them.

Also on Friday, an old mortar round killed two young boys — ages 10 and 15 — who were playing in the backyard of a farm house in Musayyib, about 60 kilometres south of Baghdad, said a police official. He spoke on condition of anonymity because he was not authorized to talk to the media.