Islamic State, which considers Shias as apostates, claimed responsibility for the attack.

A suicide truck bomb killed about 100 people, most of them Iranian Shia pilgrims, at a petrol station in the city of Hilla 100 km south of Baghdad on Thursday, police and medical sources said.

Islamic State, which considers all Shias to be apostates, claimed responsibility the attack in an online statement.

The group is also fighting off a U.S.-backed offensive on its stronghold Mosul, in northern Iraq, in which Iranian-trained Shia militias are taking part.

Medical sources said the pilgrims were en route back to Iran from the Iraqi Shia holy city of Kerbala, where they had commemorated Arbaeen, the 40th day of mourning for the killing of Imam Hussein, a grandson of the Prophet Muhammad, in the 7th century CE.

The gas station has a restaurant in its premises that is popular with travellers. Five pilgrim buses were torched by the force of the blast from the explosives-laden truck, a police official said.

Iran's Foreign Ministry condemned the attack without giving a casualties toll. Tehran will continue to support Iraq's ''relentless fight against terrorism,” Ministry spokesman Bahram Qasemi was quoted as saying by Tasnim news agency.