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A bomb attack on a bus in the southern Philippine island of Mindanao has killed nine people, officials say.

Several more were hurt when the bomb, hidden in an overhead compartment, went off as the bus travelled through Matalam in Cotabato province.

More than 50 passengers were on board at the time of the blast.

Police said it was not clear who was behind the blast, but parts of Mindanao are home to kidnappers, extortion gangs and Muslim insurgent groups.

"Eight people died on the spot and one at a local hospital where most of the bus passengers were treated for shrapnel wounds," Reuters news agency quoted Major-General Anthony Alcantara, army commander in central Mindanao, as saying.

"Our soldiers are helping the police investigate the bus explosion. We're still trying to establish the motive for the attack."

The bus company had previously received extortion letters from armed groups in the region, one official said.

Armed gangs have tried to extort money from bus companies and been blamed for attacks on vehicles and bus terminals in the past.