Nine Chinese engineers were among 13 people killed on Wednesday when a bus carrying construction workers in northwest Pakistan's mountainous region was "attacked", prompting Beijing to ask Islamabad to thoroughly investigate the incident and severely punish the perpetrators.

The incident took place in Dasu area of Upper Kohistan district of Khyber Pakhtunkhwa province where Chinese engineers and construction workers are helping Pakistan build a dam which is part of the USD 60 billion China-Pakistan Economic Corridor (CPEC), officials said.

At least 13 people, including nine Chinese nationals and two Frontier Corps soldiers, died when the bus carrying Chinese engineers and workers to the site of the under construction Dassu Dam exploded, Deputy Commissioner Upper Kohistan Muhammad Arif said in his initial statement.

The bus fell into a deep Ravine after the explosion, the official said.

The injured were admitted to nearby hospitals.

Speaking in the National Assembly, Adviser to the Prime Minister on Parliamentary Affairs Babar Awan called it a "cowardly attack" and said it would "not divert attention from the special initiatives between Pakistan and its neighbours."

Awan said he would ask Interior Minister Sheikh Rashid Ahmed to offer a briefing on the country's security situation and to keep this House in the loop about the occurrence.

No group or individual has claimed the blast yet, China's official Xinhua news agency reported from Islamabad.

In Beijing, China condemned the "bomb attack" on the bus near the Dasu hydropower plant and called on the Pakistan government to give severe punishment to the perpetrators behind it.

Asked for details of the casualties and China's reaction to the incident at a media briefing in Beijing, Chinese Foreign Ministry spokesman Zhao Lijian said China expresses its condemnation of the "bomb attack" in Pakistan resulting in the casualties of the Chinese personnel.

He, however, did not provide any official figures of casualties.

"China extends its condolences to those who died in the attack and sympathy for their families and the injured, he said.

"Security forces in Pakistan have taken actions to control the situation, properly transfer and save the injured," he said.

"The Chinese side requests the Pakistan side to look into the truth, hold the perpetrators accountable and give them severe punishment, so as to protect the security of Chinese personnel, institutions and projects," he said.

Thousands of Chinese personnel have been deployed in Pakistan to work on a host of projects being carried out under the aegis of the CPEC.

An eyewitness claimed that there was a loud noise and the bus bounced in the air and fell down, BBC Urdu service reported.

Quoting another eyewitness, it said the bus looked like flying in the air after the blast. Few seconds later, the bus fell on the ground with a loud thud.

Local people rushed to the spot where the injured were screaming, he said.

However, Pakistan's Water and Power Development Authority (Wapda), which is responsible for dam construction, said in a statement that it was an "accident".

Officials said more details would be provided after the investigation.

"The bus plunged into a deep ravine after the blast and caused heavy losses. One Chinese engineer and one soldier are missing. The rescue operation has been launched, a senior government official said.

Special Assistant to KP Chief Minister Kamran Khan Bangash said that a high level delegation has left for Upper Kohistan to ascertain the facts.

"The media is advised to avoid speculating on the matter, he said.

Bangash said that a large number of security officials were deployed to protect Chinese nationals. Those who are in critical condition are being shifted via helicopter, he said. A large number of Rescue 1122 ambulances and officials have reached the spot, he added.

Pakistan's Foreign Office in Islamabad said that the bus carrying Chinese workers plunged into a ravine after a mechanical failure resulting in leakage of gas that caused a blast.

"According to preliminary reports, nine Chinese nationals and three Pakistanis lost their lives, it said, adding that the Chinese workers and accompanying Pakistani staff were proceeding to their work place for an ongoing project.

FO said that further investigations are underway, while the local authorities are providing all possible assistance to the injured.

The Ministry of Foreign Affairs is closely in contact with the Chinese Embassy for coordination and facilitation, it said.

Pakistan and China are close friends and iron-brothers and Pakistan attaches great importance to safety and security of Chinese nationals, projects and institutions in the country, it added.