A bomb attack on a bus on the southern Philippine island of Mindanao has killed at least seven people, the army says.

菲律宾军方称，菲律宾南部棉兰老岛一辆公共汽车被炸弹袭击，至少七人死亡。

At least six others were injured when the bomb went off（爆炸，离去） as the bus travelled through the town of Matalam.

There were more than 50 passengers on board, an army spokesman said.

The company operating the bus had received extortion（勒索，敲诈） letters from armed groups in the region, the spokesman added.

Armed gangs have tried to extort¹ money from bus companies and been blamed for attacks on vehicles and bus terminals in the past.

Mindanao is home to kidnappers², extortion gangs and Muslim insurgent³（叛乱者） groups.