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. 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 Pakistans restive tribal areas along the Afghanistan border but also in the countrys cities, including the capital Islamabad. There have been signs in recent months that the Pakistan Taliban have been regrouping along the Afghan border, claiming frequent clashes with security officials. Beijing has poured billions of dollars into Pakistan in recent years to boost the countrys 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, Baloch insurgents targeted the Pakistan Stock Exchange in the commercial capital of Karachi, which is partly owned by Chinese companies.