The death toll from a double bomb attack targeting Shi’ites visiting a pilgrimage site in Damascus has climbed to 74, the Syrian Observatory for Human Rights reported on Sunday. Most of the dead were Iraqi Shi’ites who were going to visit a cemetery near the Old City of Damascus.

Footage broadcast by Syrian state TV showed two badly damaged buses with their windows blown out. The area was splattered with blood and shoes were scattered on the ground.

A Syrian alliance of jihadist groups known as Hayat Tahrir al-Sham said on Sunday it was responsible for the attack. The group said the attack on Saturday targeted Iranian-backed militias in revenge for what it said was their role in supporting President Bashar al-Assad’s “tyrannical rule”, holding them responsible for “killing and displacing” Syrians.

President al-Assad has been supported in the country’s war by Shi’ite militias from countries including Iraq, Afghanistan and Lebanon.

The attack took place at a bus station where the pilgrims had been brought to visit the nearby Bab al-Saghir cemetery, named after one of the seven gates of the Old City of Damascus.

The second blast went off some 10 minutes after the first, inflicting casualties on civil defence workers who had gathered to tend to the casualties, the Damascus correspondent for al-Manar told the station by phone.

The pilgrims were due to pray at the cemetery after visiting the Sayeda Zeinab shrine just outside Damascus, he said.

Sayeda Zeinab - the granddaughter of the Prophet Mohammad - is venerated by Shi’ites and her shrine is a site of mass pilgrimage for Shi’ites from across the world. It has also been a magnet for Shi’ite militiamen in Syria.

Iran has backed Assad in the conflict that erupted in 2011.

Last June, Islamic State claimed responsibility for bomb attacks near the Sayyida Zeinab shrine.

The Lebanese group Hezbollah is also fighting in support of Assad.