Police say a suspected bomb blast has killed at least nine people and injured more than 20 others on a bus in front of a university in the southern Philippines.

Police officer Rufina Bayarcal says a powerful blast ripped through the bus Tuesday as it was traveling near the gate of Central Mindanao University in Bukidnon province's Maramag town.

Investigators are trying to determine the source of the explosion.

A 17-year-old student aboard the bus, Ralph Quilla, said by cellphone that the blast at the back of the vehicle sent shrapnel and debris flying and wounded him in the arm.

Muslim rebels and extortion gangs have been blamed for bus bombings in the south in the past.