A further 31 Chinese nationals were injured in the blast in the Upper Kohistan district, he added

Nine Chinese workers were among 13 people killed on Wednesday when a blast on a bus sent it careering down a ravine in north-western Pakistan, officials said.

The bus was carrying Chinese engineers, surveyors and mechanical staff to the Dasu dam site in Khyber Pakhtunkhwa province, which is under construction.

"The blast sparked a fire in the engine plunging the vehicle into a ravine, a local government official told AFP, requesting not be named.

He said a further 31 Chinese nationals were injured.

A senior local police official, who also asked not to be named, confirmed the incident and said "it was a heavy blast but its nature is not known yet".

The Chinese embassy in Pakistan posted a statement on Wednesday, saying that "a certain project of a Chinese firm in Pakistan suffered an attack, which caused the deaths of Chinese nationals".

It urged Chinese firms to strengthen their security procedures.

Security of Chinese workers has long been an issue of concern in Pakistan, where Beijing has poured in tens of billions of dollars in recent years, and large numbers of Chinese nationals are now based in the country to supervise and build infrastructure projects.