At least 35 people were killed and four seriously injured after a tour bus burst into flames in Hunan province in central China on Sunday morning, state media reported. Fifty-five people were on board the coach when it smashed into a roadside barrier at about 10.20 am on Sunday morning in Yizhang county. A motorist who was behind the bus when the accident happened told the Shanghai-based news website Thepaper.cn that a survivor had told him they were all members of a tourist group from Leiyang in Hunan and were on their way to go white-water rafting. Suspected arsonist arrested after 17 killed in bus fire in northern China “After the bus crashed on guardrails on the highway, a total of two or three children, one middle-aged man and seven or eight women in their 40s got off the bus,” the report quoted the witness as saying. “Smoke came from the bus before it was engulfed by fire several minutes after the crash,” the witness said. The bus was destroyed after the fire raged for more than half an hour. WATCH: Bus fire kills 30 in central China, state media say The cause of the accident was not immediately known, but Xinhua cited sources in the Yizhang county government as suggesting that preliminary investigations showed it may have been triggered by an oil leak. Officials with the State Administration of Work Safety will also look into the possible causes of the accident and see whether the bus had been overloaded. The driver of the bus has been detained during the investigation , Xinhua said. Twenty-one people have been sent to hospital and rescuers are still searching the charred frame of the bus. Ten South Koreans killed in bus crash in northern China Adventure rafting has becomes popular in Mangshan in Yizhang county in southern Hunan and also in parts of neighbouring Guangdong province in recent years. A 5km stretch of river designated for white water rafting drops some 136 metres in Mangshan, according to the website of the area’s forest national park.