A bus carrying dozens of construction workers fell into a ravine on Wednesday in north-west Pakistan after a gas leak in the vehicle caused an explosion, killing at least 13 people including nine Chinese, Pakistani officials said.

Pakistan’s Foreign Ministry said in a statement that the Chinese workers and accompanying Pakistani staff were proceeding to their workplace when the explosion happened and the vehicle careened off the road.

The bus “plunged into a ravine after a mechanical failure resulting in leakage of gas that caused a blast … Further investigations are under way,” the statement said.

Pakistani officials were still investigating but the Chinese Embassy in Islamabad said in a statement the bus was attacked.

Pakistan’s Foreign Ministry said in its statement that nine Chinese and three Pakistanis died in the apparent accident.

However, local authorities said the death toll rose to 13 after a critically injured Chinese worker died of his injuries at a hospital.

“The Government and people of Pakistan extend sincere condolences to the families of Chinese and Pakistani workers who lost their lives in the incident,” the statement said. “We also pray for quick recovery of the injured.”

At least 36 people were also injured in the incident in the Kohistan district of Khyber Pakhtunkhwa province, said Arif Javed, a deputy district commissioner.

Chinese engineers and construction workers are helping Pakistan build a dam in Kohistan.

Mr Javed said the Pakistani and Chinese construction workers were on their way to the project site when the accident happened.

The Chinese Embassy in a statement called the incident an “attack”.

“Recently, our workers at a business on a certain project in Pakistan were attacked and have suffered deaths and injuries,” the statement said. “We have notified Chinese citizens in Pakistan to avoid venturing outside unless required by work or business and pay especial care to their safety.”

In Beijing, Foreign Ministry spokesperson Zhao Lijian said China was “shocked by and condemns the bomb attack in Pakistan’s Khyber Pakhtunkhwa province.”

“We mourn over the Chinese and Pakistani personnel killed in the attack and express sympathies to bereaved families and the wounded,” Mr Zhao said at a daily news briefing.

“We have demanded the Pakistani side get to the bottom of the incident, arrest and strictly punish the assailants as soon as possible, and earnestly protect the safety of Chinese personnel, institutions and projects in Pakistan,” he said.

Road accidents are common in Pakistan, where motorists largely disregard traffic rules and safety standards and drive on damaged roads, particularly in the mountainous terrain in the north.