CHENGDU - Fifteen were killed and six were injured after a bus overturned and fell into a ravine on Tuesday afternoon in Aba Tibetan autonomous prefecture, Southwest China's Sichuan province, local police said.

The accident happened at about 2:50 pm. The bus, carrying 21 people, fell into the ravine near a tunnel exit of the Zhegu mountain in Aba, police said.

All of the injured have been taken to a local hospital for treatment.

The bus departed from Chengdu and was on its way to Maerkang county in Sichuan when the accident occurred.

The cause of the accident is under investigation.