Two bomb blasts during Mass at a Catholic cathedral in the Philippines have killed at least 20 people and wounded some 80. Islamic militants have claimed responsibility for the act.

The first explosion went off inside the Cathedral of Our Lady of Mount Carmel and was followed by a second blast outside, which was detonated as security forces raced to the scene, officials said.

The Philippine Catholic bishops have condemned the attack as an “act of terrorism” and offered their condolences to the families of the dead and the injured.

Islamic State claimed responsibility for the bombings. Police had initially suspected the bombings were the work of Abu Sayyaf, a militant group that has pledged allegiance to Islamic State and is notorious for its brutality.

They worry that small numbers of Islamic State-linked militants from the Middle East and southeast Asia could forge an alliance with Filipino insurgents and turn the south into a breeding ground for extremists.