Iraqi security forces gather at the site of a suicide truck bomb attack, at a petrol station in the city of Hilla south of Baghdad, Iraq, November 24, 2016. Reuters/Alaa Al-Marjani BAGHDAD (Reuters) - A suicide truck bomb killed about 100 people, most of them Iranian Shi'ite pilgrims, at a petrol station in the city of Hilla 100 km (62 miles) south of Baghdad on Thursday, police and medical sources said.

Islamic State, the ultra hard-line Sunni militant group that considers all Shi'ites to be apostates, claimed responsibility the attack in an online statement.

The group also is fighting off a U.S.-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, the medical sources said.

The gas station has a restaurant on its premises that is popular with travelers. 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 Oct. 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.

U.S. 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 U.S. State Department is in close contact with Iraqi authorities, department spokesman John Kirby said in a statement.

(Reporting by Saif Hameed and Mohammed el-Sherif; Additional reporting by Robert Iafolla in Washington; Writing by Maher Chmaytelli; Editing by Toby Chopra and Bill Trott)