MANSEHRA, Pakistan (AP) -- A bus carrying Chinese and Pakistani construction workers on a slippery mountainous road in northwest Pakistan fell into a ravine Wednesday, killing at least 13 people, including nine Chinese nationals, officials said.
At least 36 people were also injured in the apparent accident in the Kohistan district of Khyber Pakhtunkhwa province, said Arif Javed, a deputy district commissioner.
The incident happened on a wet road following overnight rain but Pakistani officials were still investigating and the Chinese Embassy said in a statement the bus was attacked.
Chinese engineers and construction workers are helping Pakistan build a dam in Kohistan. Javed said the Pakistani and Chinese construction workers were on their way to the project site when the accident happened.
Asim Abbasi, an assistant commissioner in Kohistan, said authorities believe the incident was an accident but were investigating whether there was some type of explosion in the bus. He said a gas cylinder may have exploded and it was possible that explosive material was in the vehicle. Explosives are often used by engineers in construction projects.
"So far, we are not sure whether there was a blast in the bus but the bus fell into a ravine and it seems it was an accident," he said.
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," 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," Zhao said.
Kamran Bangash, a government spokesman for Khyber Pakhtunkhwa province, asked media not to speculate because officers were still investigating. He said helicopters were transporting critically injured persons to hospitals for treatment.
Bangash said investigators would soon brief the media about the cause of the incident. His comments came after local media quoted unnamed sources to report that there was an explosion in the bus carrying Chinese and Pakistani construction workers and security officials.
Road accidents are common in Pakistan, where motorists largely disregard traffic rules and safety standards on damaged roads, particularly in the mountainous terrain in the north.