Rescue workers and onlookers gather around a wreck after a bus plunged into a ravine following an explosion, that killed 12 people including 9 Chinese workers, in Kohistan district of Khyber Pakhtunkhwa province on July 14. Photo: AFP / STRINGER

China on Saturday said it has sent investigators to Pakistan to probe an explosion on a bus that killed 12 people including nine Chinese nationals, calling the blast a “terrorist attack.”

The bus was carrying around 40 Chinese engineers, surveyors and mechanical staff to a hydropower dam construction site in the northwestern Khyber Pakhtunkhwa province on Wednesday.

The other three people killed in the explosion were Pakistanis, while around 28 were injured.

Islamabad previously blamed the blast on a mechanical failure that led to a gas leak, but China has called it a bombing.

“China and Pakistan will work together to find out the truth,” Chinese public security minister Zhao Kezhi said in a statement after a phone call with his Pakistani counterpart.

Beijing has “sent technical experts in criminal investigation to Pakistan to assist in the investigation,” he added.

He called on Islamabad to strengthen security for Chinese nationals in the country.

On Wednesday China urged Pakistan to “severely punish” those responsible for the bus explosion and called on authorities to “earnestly protect” Chinese nationals and projects.

Islamabad is Beijing’s closest regional ally, but the security of Chinese workers in Pakistan has long been of concern.

Large numbers of them are based in the country to supervise and build construction projects.

Beijing has poured billions of dollars into Pakistan in recent years to boost the nation’s infrastructure.

In April a suicide blast at a luxury hotel hosting the Chinese ambassador in southwest Balochistan killed four people and injured dozens. The ambassador was unhurt.