Improvised explosives set the vehicles ablaze one after the other as they were being driven near the town of Isulan on Saturday, said the provincial police chief, Senior Superintendent Rolen Balquin.
Four passengers and the driver and conductor of one of the vehicles were slightly injured as firemen were called out to put out the blazes, said regional military spokesman Colonel Prudencio Asto.
“Initial findings of the explosives and ordnance team revealed that the explosions were caused by an improvised explosive device made out of ammonium nitrate, fuel and oil with common nails,” Asto told reporters.
Balquin said they had no immediate suspects, and it was unclear if the buses which were on regular runs between the cities of Cotabato and Tacurong, were carrying the explosive devices before they went off.
The blasts occurred in a section of the southern island of Mindanao that had been troubled by a decades-old Muslim insurgency and high crime, in which transport firms are often targeted for extortion.
Moro Islamic Liberation Front rebels in October signed a preliminary peace deal with the government that committed both sides to create a Muslim self-rule area for the Islamic minority of the Catholic nation.
The rebels have been observing a ceasefire since then.