A blast on a bus killed 13 people in north Pakistan on Wednesday, including nine Chinese nationals in what Beijing said was a bomb attack but Islamabad called a vehicle failure.

Two Pakistani soldiers were also among the dead after the explosion sent the bus over a ravine, local government and police sources told Reuters.

Chinese engineers and Pakistani construction workers have for several years been working on hydroelectric projects as part of Beijing's Belt and Road Initiative in the western province of Khyber-Paktunkhwa, where the blast occurred.

China's embassy in Pakistan confirmed that nine of its nationals died. Terming the explosion a bomb attack but not giving more details, the Chinese foreign ministry offered condolences and urged both a thorough investigation and protection of its personnel and projects.

Pakistan's foreign ministry said a mechanical failure caused a gas leak which led to the explosion.

However, the province's top police official, Inspector General Moazzam Jah Ansari, earlier told Reuters foul play was suspected.“Looks like sabotage,” he said.

A senior administrative officer of the Hazara region, who asked not to be named, said the bus was carrying more than 30 Chinese engineers to the Dasu dam in Upper Kohistan.

The Dasu hydroelectric project is part of the China-Pakistan Economic Corridor (CPEC), a $65 billion investment plan aiming to link western China to the southern Pakistani port of Gwadar.

”This is clearly an act of terrorism that has been carefully planned and was supported by information,” said an editorial in Global Times, a Chinese tabloid run by the ruling Communist Party’s official People’s Daily, calling it the most serious attack on Chinese nationals in recent years.

Chinese working in Pakistan have been attacked previously, particularly in the south-western province of Balochistan where separatist militants have waged an insurgency against authorities in a state where China develops mines and a port.

The separatists have also attacked the Chinese consulate in the southern city of Karachi. Additionally, Islamist militants have also previously targeted Chinese nationals.

Using an air ambulance, rescuers took the injured, including Chinese engineers, to a hospital in Dasu, about 10 km (6 miles) from the blast site, authorities said.

”Police and the bomb disposal squad are at the site,” added regional official Arif Khan Yousufzai outside the hospital, adding that an investigation was awaited to ascertain details.

Pakistan's foreign ministry said it was in close contact with the Chinese embassy in Islamabad, adding in a statement: ”Pakistan attaches great importance to the safety and security of Chinese nationals, projects and institutions.