Twin bombings Sunday at a cathedral in the restive southern Philippines have killed 20 people and wounded 81. Police say earlier death tolls were incorrect because of double counting.
The government, in a statement, has promised to hunt down the attackers "until every killer in brought to justice and put behind bars." The statement said, "the ruthless perpetrators" will be shown "no mercy."
The first bomb exploded during Mass inside the cathedral in Jolo, in the province of Sulu. As military and police forces rushed in to help, the second bomb went off.
Philippine President Rodrigo Duterte condemned the bombing as an "act of terrorism."
United Nations Secretary-General Antonio Guterres called "for the perpetrators of these crimes to be swiftly brought to justice."
Late Sunday, Islamic State's news agency Amaq said the terrorist group claimed responsibility for the blasts.
Jolo is situated in a mostly Muslim region of the predominantly Catholic Philippines.
The blasts come just days after the region held an autonomy referendum, which was overwhelmingly approved. Sulu province voters, however, rejected the autonomy vote but it still will be part of the new entity.
Sulu is home to the Islamist militant group Abu Sayyaf, which has been blamed for a number of deadly bombings.