At least 12 people, including 9 Chinese citizens, have been killed after a bus carrying construction workers in Pakistan’s far north plunged into a ravine following an explosion, the Pakistani foreign ministry said.

The incident took place as a bus carried workers to the Dasu hydropower project in northern Khyber Pakhtunkhwa province, about 190km north of the capital, Islamabad, on Wednesday morning.

The cause of the explosion was not immediately clear, with the Pakistani foreign ministry saying it took place due to a “mechanical failure”, while a Chinese foreign ministry spokesperson referred to it as an “attack”.