Three people were wounded in a home-made bomb explosion inside a passenger bus in south Philippines on Monday, a local official said.

The Rural Transit Incorporated bus, with license plate number KVS700, was leaving a depot in Kabacan township at 2 p.m. local time (0600 GMT) when the improvised bomb exploded, Governor Jesus Sacdalan of North Cotabato province told Xinhua by phone.

"We are still determining the kind of explosive used by the attackers. We believe that the attack was carried out by extortionists blamed for the series of attacks in the region," Sacdalan said.

Regional military spokesman Major Randolph Cabangbang said the blast victims were rushed to the town's specialist hospital.

It was the third attack of the day. Two previous bombs, also improvised, exploded near a bridge in the southern Philippine city of Iligan early Monday morning, causing heavy damages to the bridge and nearby houses, but no one was found injured.