At least 50 others were injured in the explosion in the city of Marawi.
These government photos shared online show the immediate aftermath of Sunday's attack, which took place in a university gym.
Although there appears to be no damage to the building, burn marks can be seen in the center of the room.
President Ferdinand Marcos Jr has condemned the incident as a "heinous act perpetrated by foreign terrorists".
At a press conference, Philippine defense officials vowed to find those responsible.
"We shall leave no stone unturned in order to root out the perpetrators of this heinous crime. Let us remember that this was done in a religious occasion in Marawi and the intent really to foment terrorist activity, to foment confusion, chaos is an apparent initial of a terrorist activity."
Defense Secretary Gilberto Teodoro further confirmed there were "strong indications of a foreign element" in the bombing, but refused to elaborate so as not to compromise ongoing investigation.
In 2017 the city of Marawi was besieged for five months by Islamist militants.
During the battle between those fighters and Philippine forces, more than a thousand people were killed including civilians.
Sunday morning's attack follows a series of military operations against local pro-Islamic State groups in the southern Philippines.
Both the police and the military have strengthened security in the country's south and around the capital, Manila.