A suicide truck bomb has killed about 100 people, most of them Iranian Shi'ite pilgrims, at a petrol station in a city south of Baghdad.
Islamic State, the ultra hard-line Sunni militant group that considers all Shi'ites to be apostates, has claimed responsibility for the attack on Thursday in an online statement.
The group also is fighting off a US-backed offensive on its stronghold Mosul, in northern Iraq, in which Iranian-trained Shi'ite militias are taking part.
The pilgrims were en route back to Iran from the Iraqi Shi'ite 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 Mohammad, in the 7th century AD, a medical sources said.
The gas station in the the city of Hilla, 100km from Baghdad, has a restaurant on its premises that is popular with travellers. Five pilgrim buses were set afire by the blast from the explosives-laden truck, a police official said.
In recent months Islamic State has intensified attacks in areas out of its control in efforts to weaken the offensive launched on October 17 to retake Mosul, the last major Iraqi city under Islamic State control.
Iran's Foreign Ministry condemned the attack without giving a casualty toll. Tehran will continue to support Iraq's "relentless fight against terrorism," ministry spokesman Bahram Qasemi was quoted as saying by Tasnim news agency.
US officials condemned the attack.
"The United States remains steadfast in its partnership with the Iraqi people and government and this attack only serves to strengthen our resolve in defeating ISIL," National Security Council spokesman Ned Price said in a statement.
The US State Department is in close contact with Iraqi authorities, department spokesman John Kirby said in a statement.
