Thirty-two people were wounded when two home-made bombs exploded during a boxing match in a town, an army spokesperson said on Thursday.
The victims were watching the boxing competition organised as part of an annual festival at a public plaza in the town of Hilongos, Philippine in the province of Leyte when the blast occurred.
Sixteen of those hurt were still in hospital, while the rest were discharged after being treated, said Colonel Benjamin Hao, citing a joint statement by the police and the military.
The improvised explosive devices were made from 81-millimetre mortars, based on fragments recovered from the blast site, and were detonated with the use of a mobile phone, Hao said.
Police are looking into the possibility that the bombing was in retaliation for the arrest of a woman during a drugs raid in October in Hilongos, said First Lieutenant Cherry Junio, a regional army spokesperson.
The administration of President Rodrigo Duterte has launched an aggressive war against illegal drugs in the Philippines since he took office on June 30.
Under the campaign, 2,124 drug suspects have been killed in police operations from July 1 to December 18, while authorities are investigating 2,928 deaths in the same period for possible involvement in illegal drugs.
Muslim separatist rebels in the southern Philippines are known to use mortars to make home-made bombs for various attacks but Leyte has not been a target of such bombings in the past.
The bombing comes days after 12 people were injured by a grenade explosion outside a Catholic church in the southern Philippines during a Christmas Eve mass.
No group has claimed responsibility for the church attack in the province of North Cotabato, but Islamist militants and local terrorist groups are known to be operating in the area.