At least 22 people were killed Tuesday when a bus slipped off a mountain gravel road in eastern Nepal and rolled down a hill, police said.

The bus plunged off the narrow road near Barkhetar, 200 kilometers (125 miles) east of the capital, Kathmandu, and rolled down a hill for nearly 200 meters (650 feet), said police official Umesh Khadka.

Another police official, Sangita Baniya, said that the death toll had risen to 22 by Tuesday evening and that all of the 20 people injured in the accident had been taken to hospitals for treatment.

Police are investigating the cause of the accident.

Nepal is mostly covered with mountains, and most roads are narrow with hairpin bends, with only gravel roads linking mountain villages.

Most accidents in Nepal are blamed on poorly maintained vehicles and road conditions. Buses are crowded and people often travel on the roofs.