A passenger bus tumbled off a mountain highway in northern Pakistan (search) Sunday, leaving three people dead and at least 20 others missing and feared killed, police said.

About 17 people, who were injured, were pulled out of the bus which fell into a fast flowing stream in Kohistan, said Waheed-ur Rahman, an official at the police control room in Kohistan, a rugged district about,100 miles north of the capital Islamabad.

At least 20 other passengers were missing and believed trapped in the debris of the bus or their bodies washed away, Rahman said.

Army troops in the area have been called to help and speed up the rescue work. "The water is deep and we fear those missing may have been killed," Rahman.

A cause for the crash was not immediately known.

The bus with some 42 people on board was heading from Skardu (search), a town in the Himalayan region, to Rawalpindi, a city near Islamabad. The highway the bus was traveling along connects Pakistan with China in the north.

Road accidents are common in Pakistan where most roads are poorly maintained and drivers of public transport vehicles are often not properly trained and widely disregard traffic rules.