Five of those killed were kindergarten children travelling in a minibus with seven other classmates when their vehicle was engulfed by floodwaters from the Qingjiang River, a tributary of the Yangtze River, in Enshi city .

The 12 children from the local Wangwang Kindergarten were believed to have been on their way home when the Dalongtan reservoir dam, which is more than 50 metres high, collapsed.

A massive rescue operation led by Hubei governor Luo Qingquan recovered the bodies of the five children and their teacher on Friday.

The other seven children are still missing, along with the driver of their bus and four dam workers, who were washed away when they returned to the site to retrieve their clothes, the China Daily reported.

Rescuers believe there is little hope of finding any survivors as rescue efforts have been hampered by torrential rain, which authorities are blaming for the disaster.

The area recorded about 66mm of rain the day before the reservoir collapsed, the heaviest rainfall recorded so far this year, according to the local meteorological department. But some residents have challenged this explanation, mainland media reported.

"How is it possible that a reservoir can be so weak as to collapse after a rainstorm lasting only one day?" one resident was quoted as saying by China Central Television.

Tang Changyuan, an official in Dalongtan village where the dam is located, told the Guangzhou-based Southern Metropolis News he was not surprised that the dam collapsed because it was not built with the proper materials.

"The first 40 metres [in height] of the dam built with cement was strong, but things changed during the second phase of the construction. They did not use cement to build the remaining 10-odd metres of the dam. Instead, they used clay and stones," said Mr Tang, who worked on the project.

He said the dam had survived a much smaller flood last month when leaks and a minor collapse were identified.

On Friday, Premier Wen Jiabao urged the local government to spare no efforts in attempts to rescue the victims and ordered an immediate investigation.

The State Flood Control and Drought Relief Headquarters has also sent an investigation team to Enshi headed by deputy director Cheng Dianlong .

In the wake of the accident, authorities yesterday urged eight provinces and cities along the Yangtze River, including Shanghai and Jiangsu, to ensure all construction projects were safe during the flood season, Xinhua reported.