Nine Chinese workers were among 13 people killed on Wednesday when a blast on a bus sent it careering down a ravine in northwestern Pakistan, government and police officials said.
While Pakistani officials said the nature of the blast was under investigation, the Chinese embassy in Islamabad said its nationals had come under "attack".
The bus was carrying Chinese engineers, surveyors and mechanical staff to the Dasu dam construction site in Khyber Pakhtunkhwa province, a local government official who did not want to be named told AFP.
"The blast sparked a fire in the engine plunging the vehicle into a ravine," he said.
He said a further 28 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 security of Chinese workers has long been an issue of concern in Pakistan. Large numbers of them are based in the country to supervise and build infrastructure projects.
In a statement on Wednesday, the Chinese embassy said 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.
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