Twelve of the missing are children, whose school minibus was engulfed as they headed home on Thursday.

So far only one body has been found, that of a three-year-old child, about 1 kilometre downstream.

The accident happened in Hubei province near the town of Enshi, where a reservoir dam is being constructed.

Xinhua said local government officials had mobilized more than 5,000 people, including 500 soldiers and armed police, to search downstream along the Qingjiang river, a tributary of the Yangtze River.

It said the Dalongtan reservoir was still under construction when rainstorms on Wednesday and Thursday brought torrents of water down into the reservoir and broke the cofferdam, a temporary structure designed to hold water back while construction goes on.