Cotabato City (Philippines), Oct 21 (DPA) Nine people were killed Thursday in a bombing inside a passenger bus in the southern Philippines, the military and police said.

The bus was travelling along a national highway in Matalam town in North Cotabato province, 930 km south of Manila, when the blast occurred.

Lieutenant Colonel Randolph Cabangbang, a regional military spokesman, said nine passengers were also injured in the attack, believed to have been perpetrated by extortionists.

"The bus company has long been receiving extortion letters from armed groups operating in the region," he said.

Chief Superintendent Gil Meneses, a regional police commander, said more than 60 people were aboard the Rural Transit bus when the explosion occurred.

He said the bus was travelling to nearby Tacurong City from Cagayan de Oro when it made a stopover in Matalam town.

"The bomb must have been planted in the bus during the stopover," Meneses said.

Initial investigation showed the bomb was placed in a bag left in the overhead baggage compartment at the rear portion of the bus, according to Senior Superintendent Cornelio Salinas.

Salinas, the provincial police commander, said the roof of the bus was torn off by the blast.

"We are looking at extortion," he said, but added that authorities were not yet discounting the possibility that terrorism could be involved.

Rural Transit, which owns the bombed bus, has been a target of extortion groups in the area. In the past, other buses of the company had been attacked for failing to pay protection money.

Islamic militants also operate in the province and have been blamed in the past for bombing public utilities.