Eleven people have been killed after a bomb exploded aboard a bus in the southern Philippines on Tuesday, a military spokesman has said.

The bus was travelling through the town of Maramag in the strife-torn island of Mindanao when the bomb went off, said Major Christian Uy.

At least 21 other people were also injured in the blast, the authorities said.

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 Jiselle Longakit, the provincial police spokeswoman. The type of bomb is still to be determined, she added.

The explosion was the latest in a series of attacks on vehicles owned by the Rural Transit company.

Last month, a bomb exploded aboard a Rural Transit bus in Mindanao, leaving four people wounded. Ten people were killed when a bomb exploded aboard a bus operated by the same company on Mindanao in 2010.

Muslim rebels were blamed for carrying out the previous bombings as part of an extortion scheme.

Armed groups have proliferated in the southern Philippines since Muslim rebels began fighting in the 1970s to set up an Islamic state in that part of the largely Christian Philippines.

Source: Agencies