Some 2,000 Chinese troops were sent today to repair "extremely dangerous" cracks in a dam upstream of an earthquake-hit city where 500,000 people live.
Officials warned that Dujiangyan "would be swamped" if the Zipingpu reservoir were to breach the hydroelectric dam, five miles upstream of the south-western city.
The state news agency, Xinhua, said 2,000 troops had been sent to help to repair the dam.
Earlier, engineers released water from the reservoir to relieve pressure on the dam, after cracks appeared on its surface.
Speaking to Reuters, He Biao, the deputy Communist party chief of Aba prefecture, said: "If the danger intensified, it could affect some power stations downstream. This is an extremely dangerous situation."
An official from the ministry of water resources warned: "If Zipingpu develops a serious safety problem, it could bring disaster to Dujiangyan."
The city, just north of the provincial capital, Chengdu, was rattled by Monday's earthquake, with buildings collapsing and services cut off.
Dujiangyan claims to have the world's oldest operating irrigation system. The third century dikes and weirs on the Min river suffered only minor damage from the earthquake. But it is feared the system would collapse under the weight of water if the modern dam burst upstream.
The water minister, Chen Lie, urged local officials to evacuate people if further problems emerged.
Yesterday authorities pointed out that the earthquake had not damaged the huge Three Gorges dam, which is still incomplete. The quake registered a magnitude of four in the dam area, which is 600 miles from the epicentre of the quake, where it registered 7.9.
The Three Gorges dam is designed to withstand earthquakes up to seven in magnitude. However, one of the many criticisms made of the dam was that its sheer size could trigger earthquakes.
In 2002 Bill McGuire, the director of London's Benfield Greig Hazard Research Centre, said: "There's no question that if you dig a big enough reservoir, you're going to get earthquakes. The Three Gorges dam in China is going to be a big problem."
However, today McGuire said: "Reservoir-related loading effects can trigger the activation of local faults but not those at a significant distance.
"This earthquake is purely tectonic and fits with the expected return period for the region [of 50 to 100 years]."
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