Two thousand soldiers have been sent to plug cracks in a dam upriver from the earthquake-stricken town of Dujiangyan.

The Ministry of Water Resources called for the urgent protection of the Zipingpu reservoir, saying that Dujiangyan, which has about 600,000 residents, would be swamped if the dam failed.

The Zipingpu dam is among the most modern in China but was built despite warnings it lay close to a major earthquake fault line. Planning for the dam was in its early stages in 2000 when seismologists from China’s Earthquake Bureau warned that it could be at risk.

Earthquake protection would have been built into the design of the dam, which was constructed in an area where 9,000 people died in 1933 amid landslides caused by an earthquake.

Nevertheless, the enormous reservoir has the capacity to hold 1.2 billion cubic metres of water and if it were to collapse the consequences would be disastrous.

“This is a seriously big dam,” said Ian Cluckie, a professor of hydrology and water management at the University of Bristol.

“If it were to go pop it would be absolutely diabolically catastrophically beyond belief.”

There were concerns last night that water levels in the dam were still rising, suggesting that there was some sort of blockage preventing the Chinese authorities from draining it.

“If there’s rising water the normal way of getting water out of the dam isn’t working. They could just have a major problem with this one. About the only thing you can contemplate doing in such circumstances is dropping a load of sand, concrete or soil into it and hoping it fills a crack.”

The earthquake had also caused a hydroelectric generating unit at Zipingpu to collapse. It went into operation only in 2006 as part of a programme to develop China’s poorer western regions.

Wen Jiabao, the Chinese Premier, travelled to Beichuan and Wenchuan to direct rescue efforts, comfort survivors and promise that help would come as swiftly as possible. “Your pain is our pain,” he told survivors in Wenchuan. More than 50 planes had already been diverted to try to bring out the injured and more would be found, he promised.

Mr Wen ordered an additional 3,000 medical staff to the province. Flights to airports across China have been disrupted by the huge operation to mobilise troops and send in aid.

Columns of military lorries carrying hundreds of soldiers drove towards Beichuan and Wenchuan through the lush green hills of Sichuan. More than 100,000 troops have been deployed since Monday’s tremor. Television footage from near the epicentre showed shattered buildings, roads split in two and whole mountainsides sheared off.

Two hydroelectric power stations in Maoxian county, where 7,000 residents and tourists remain stranded near the epicentre, have been seriously damaged. Authorities have given warning that dams in the area could burst. The National Development Reform Commission, China’s top economic planning body, said the earthquake had damaged 391 dams, most of them small.

It was regarded as fortunate that the Zipingpu dam, completed in late 2006, was only about half-full when the earthquake struck because it meant the pressure was much lower than it might have been.

When cracks appear in dams the best way to reduce the risk of a catastrophic failure is to lower the pressure by letting the water out. This was the technique employed last summer when the Ulley dam close to Sheffield threatened to give way during the floods.

Chinese expertise in dam construction, however, is among the best in the world and the country has a reputation for building some of the most reliable and structurally sound reservoirs.

There are 22,000 dams of varying sizes in China, about half the world’s total, and many of them were built decades ago.

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