Heavy equipment has been flown in by helicopter to the remote mountain area.


Officials hope the operation to drain the lake will prevent the dam from bursting and causing massive flooding.


The work has been made all the more urgent with aftershocks continuing to trigger further landslides and forecasts of heavy rains threatening to put more pressure on the lakes.


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As of late Wednesday, the surface of the lake was edging closer to the top of the dam, reaching just 2m below the lowest point of the blockage, according to the Xinhua news agency.


A contingency plan has been put in place to evacuate some 1.3 million more people from towns and villages downstream if the operation to divert the water does not go according to plan.


The number of dead from last month's quake has topped 69,000 with nearly 18,000 more still listed as missing and 5 million people left homeless.