A homemade bomb exploded along a highway in a southern Philippine city on Tuesday, but no one was hurt in the third bomb attack this week in the area, a local official said.
The bomb, made from a 60-millimetre mortar shell, went off as a passenger bus passed on the highway in the village of Tamontaka in Cotabato City, 960 kilometres south of Manila, BBC reported.
"It would appear that the bomb was intended for the Husky Bus Company," Cotabato City Mayor Muslimin Sema said.
On Sunday, a bomb exploded at a terminal of the Husky Bus Company in Cotabato City in a pre-dawn attack that damaged two buses but injured no one.
Later in the evening, a bomb exploded on a pedestrian overpass bridge in the nearby city of Kidapawan, injuring nine people, police said.
Police investigators said extortion could be the motives for the bombings.
But an army spokesman said Muslim separatist rebels could also be behind the attacks.
Extortion groups have in the past bombed passenger buses and terminals in the southern Philippines when owners refused to pay up, killing dozens of people.
Islamic militants have also been blamed for previous attacks on transportation systems in the southern Philippines, home to a decades-old Muslim insurgency.