0629 GMT October 17, 2021
China described it as a "bombing" and an "attack", although Pakistani authorities said the nature of the blast was under investigation, according to AFP.
The bus was carrying Chinese engineers, surveyors and mechanical staff to the Dasu dam construction site in Khyber Pakhtunkhwa Province, said a local government official who did not want to be named.
"The blast sparked a fire in the engine plunging the vehicle into a ravine," he said, adding 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 been an issue of concern in Pakistan. Large numbers of them are based in the country to supervise and build infrastructure projects.
China's Foreign Ministry spokesman Zhao Lijian expressed his "shock and condemnation over the bombing", urging Pakistan to "severely punish" the perpetrators and "earnestly protect" Chinese nationals and interests.
The Chinese Embassy in Islamabad 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 in the country to strengthen their security procedures.
In April, the Pakistani Taliban claimed responsibility for a deadly suicide blast at a luxury hotel hosting the Chinese ambassador, who was unhurt, in southwest Balochistan.
The group has recently claimed a string of attacks not only in Pakistan's restive tribal areas along the Afghanistan border but also in the country's cities, including the capital Islamabad.
Beijing has poured billions of dollars into Pakistan in recent years to boost the country's infrastructure.
But Chinese-funded projects have sparked resentment, particularly among separatist groups, who say locals see little benefit, with most jobs going to outsiders.
In 2019, gunmen stormed a luxury hotel in Balochistan overlooking a flagship Chinese-backed project – the deep-water seaport in Gwadar that gives China strategic access to the Arabian Sea – killing at least eight people.
And last June, terrorists targeted the Pakistan Stock Exchange in the commercial capital of Karachi, which is partly owned by Chinese companies.