Chinese troops are carving a third drainage channel into the unstable dam holding back a "quake lake", as water levels rise and aftershocks send more debris into the water, state media reported today.
Soldiers are also using short-range missiles and dynamite to blast apart boulders blocking the first channel, to speed up the flow of water, the official Xinhua agency said.
A string of powerful aftershocks on Sunday and Monday have made the work even more challenging.
The Tangjiashan lake rose nearly a metre in 24 hours to this morning, and italready stood over two metres above the drainage channel even though it only began operating on Saturday.
The massive barrier of rocks and mud was created in mid-May when a major quake shook China's southwestern Sichuan province, sending part of a mountainside crashing into the Tongkou river.
If the dam gives way it will create a floodwave that could threaten up to 1.3 million people downstream, many already forced from their homes by the quake. Over 250,000 have been evacuated as a precaution.
The arrival of China's rainy season is an additional worry.
Separately, the State Council Information Office said the death toll from the 7.9 magnitude quake had risen slightly to 69,142 as of midday Monday, with 17,551 others still reported missing. Some 374,065 people had been injured.