At least 26 people have been killed and 13 injured after a bus plunged down a steep ravine and landed in a river in Indonesia.About 50 passengers were on board the bus in South Sumatra province when it plunged some 150m (500ft) on Monday night.The bus had left Bengkulu city and was heading for Palembang, a journey of several hours.About 120 rescuers are searching for survivors.The cause of the accident is being investigated.The accident happened in the Liku Lematang area at about 23:15 local time (16:15 GMT).Pagar Alam police chief Dolly Gumara said officers were currently prioritising evacuating survivors.The police chief also called for victims' families to identify their relatives at the hospital.Roads around the site of the accident have been closed and police say they have issued warnings to motorists.Serious road accidents are common in Indonesia with roads and vehicles often poorly maintained.Last year, 27 people died after a tourist bus ran off a road and overturned in West Java province. It had been taking 40 passengers to visit thermal springs.Two months later, two accidents in West Java killed at least 15 people.