KATHMANDU, Dec. 21 (Xinhua) -- At least 16 people were killed, 14 sustained injuries and seven others went missing after a bus returning from a picnic drove off a highway in Dang district of the western Nepal on Friday, local authorities said here.

"Sixteen people were killed instantly, 14 others were injured and seven went missing as the bus veered off the highway and plunged some 500 meters down. We have been carrying out rescue operation in coordination with local residents and security personnel," Chief District Officer Govinda Prasad Rijal told Xinhua by phone from the crash site.

Rijal said that most of the passengers in the bus were students and teachers from a local school returning to their homes after a picnic in Salyan district.

"We have yet to ascertain the identities of those killed and injured in the accident. But it seems most of the victims were students," he told Xinhua.

The injured are receiving treatment in a local hospital. Local authorities have yet to find out the cause behind the accident.

Poor condition of roads, drunk-driving, over-speeding, and ill-maintained vehicles are often blamed for frequent road accidents in Nepal.