The blast occurred in the evening when the victims were having dinner in the restaurant, Philippine newspaper Inquirer reported, citing local police.

"We are still determining the motive [of the crime], we are reviewing the CCTV, but initially, it was a quarrel among the restobar clients," the newspaper quoted a police representative as saying.

No one took responsibility for the explosion, the newspaper said, adding that the police found fragments of an MK2 hand grenade at the crime scene.

Terrorist attacks and clashes between the army and Islamist groups occasionally occur in the province of Cotabato, located on the southern Philippine island of Mindanao.

The Islamists are blamed for this year's bombings in Koronadal, Tacurong and other cities in the Philippines.