It was not immediately clear whether the blast was the result of a roadside device or something planted inside the bus.

Inspector General Moazzam Jah Ansari, the top police official of Khyber-Paktunkhwa, the province in which the incident occurred, confirmed the death of six Chinese nationals, two soldiers and two locals.

“The bus plunged into a deep ravine after the blast and caused heavy losses. One Chinese engineer and one soldier are missing. The rescue operation is launched and the entire government machinery has been mobilised to rescue the injured by air ambulance,” a senior government official told Reuters on condition of anonymity.

Ansari told Reuters that police were investigating.

“Looks like sabotage,” Ansari said as he was departing on a helicopter for the site.

At least three other officials confirmed to Reuters that a blast hit the bus.

Another senior police official told Reuters that the toll had already risen to 13 dead, including nine Chinese nationals and two soldiers.

A senior administrative officer of the Hazara region told Reuters the bus was carrying more than 30 Chinese engineers to the site of the Dasu dam in Upper Kohistan.

The Dasu hydroelectric project is part of the China-Pakistan Economic Corridor (CPEC), a $65 billion investment plan under Beijing’s Belt and Road Initiative that is aimed at connecting western China to the Gwadar sea port in southern Pakistan.

Chinese engineers and Pakistani construction workers have been working on the Dasu hydroelectric project and several others for several years in the region where the blast took place.