DAVAO, Philippines (AFP) - Eleven people including many high school students were killed when a bomb exploded aboard a bus in the southern Philippines on Tuesday, military spokesmen said.

The bus operated by the Rural Transit company was travelling through the town of Maramag in the strife-torn island of Mindanao when the bomb went off, said regional spokesman Major Christian Uy.

The blast also injured 21 people, he told AFP.

Many of the victims were high school students who had just boarded the bus as it passed a school in the largely-Christian town, said Lieutenant Norman Tagros, spokesman for a local infantry brigade.

The bomb went off almost immediately after the students got on, he said.

The bomb was apparently hidden in a sack that was placed in the vehicle's overhead baggage rack, said Inspector Jiselle Longakit, the provincial police spokeswoman.

The type of bomb is still to be determined, she said.

Tagros said extortion was being eyed as the likely motive for the bombing, the latest in a series of attacks on the bus company.

Last month, a bomb exploded aboard a Rural Transit bus in Mindanao, leaving four people injured, an attack police believe was orchestrated by the Bangsamoro Islamic Freedom Fighters (BIFF), Longakit said.

The BIFF is a breakaway Muslim extremist group opposed to peace talks between the government and the main Muslim rebel group, the Moro Islamic Liberation Front (MILF).

Ten people were also killed when a bomb exploded aboard a bus operated by the same company on Mindanao in 2010, which was blamed on an extortion scheme also linked to Muslim extremists.

Armed groups have proliferated in the southern Philippines since Muslim guerillas began fighting in the 1970s to set up a separate state for the Muslim minority of the largely Christian Philippines.

Among the Muslim outlaw groups still active in the south is the BIFF and the Abu Sayyaf, an Al-Qaeda-linked group blamed for the worst terror attacks in Philippine history.

Both groups have been accused of carrying out bombings, often using improvised explosives.

The main Islamic rebel group, the MILF, signed a peace agreement with the government in March but the Abu Sayyaf and the BIFF are not part of the accord.