Nine Chinese engineers were among 13 people killed on Wednesday when a bus carrying construction workers in northwest Pakistan’s mountainous region was “attacked”, according to officials and eyewitnesses.

The incident took place in Dasu in Khyber Pakhtunkhwa province where Chinese engineers and construction workers are helping build a dam which is part of the $60 billion China-Pakistan Economic Corridor, officials said. The bus fell into a deep ravine after an explosion, an official said. However, the Water and Power Development Authority which is responsible for dam construction, said in a statement it was an “accident”.