Thousands of Chinese dams have been described as "time bombs" by Chinese officials.

They said more than one-third of the country's estimated 85,000 dams are defective and need urgent repairs.

Experts at the Ministry of Water Resources, quoted by the official China Daily newspaper, said the dams could cause fatal floods

They said some 33,000 dams were old and poorly built.

The newspaper said the ministry was hoping for approval from the central authorities for as much as $4bn to carry out the repairs, but said it would be difficult to raise the money quickly.

Quick fix solutions

Concern about the safety of China's dams was highlighted during last year's disastrous floods, when at least 3,000 people died. Many of China's estimated dams are used to control the flow of rivers.

The paper noted that many of the dams were built between the late 1950s and late 1950, the period of Maoist political movements, when it said technological criteria were often ignored in favour of quick results.

It said 3,200 dams had failed since China's communist revolution, one of the worst cases occurring in Henan Province in 1975 when two dams collapsed.

The paper said 20,000 people drowned, though human rights groups quoting internal Chinese reports have said the figure was far higher.

Risks not known

The experts warned that many local officials were not fully aware of the risks posed by sub-standard reservoirs.

The article follows unusually direct criticisms in official media of China's largest dam project, the giant Three Gorges hydro-electric dam currently under construction on the Yangtze.