China readied on Friday to ease pressure on a swollen "quake lake" threatening hundreds of thousands of people downstream in the southwestern province of Sichuan as the water level quickly rises toward a man-made sluice.

The Tangjiashan lake is the largest of the more than 30 quake lakes formed when the May 12 earthquake triggered landslides that blocked rivers, raising fears of a secondary disaster after the tremor that killed more than 69,000.

Water behind Tangjiashan's natural mud-and-rock dam rose to 739.52 metres (2,440 feet) above sea level at 4 a.m. GMT (5 a.m. British time), only 48 centimetres from the sluice, state television said, which meant partial discharge of floodwaters building up behind could occur within hours.

But the chance of the unstable dam collapsing was also increasing "under the continuous influences of aftershocks, rainfall and other uncertain factors", the official Xinhua news agency said.

"Quake lakes" burst weeks after two powerful earthquakes hit the same area in 1786 and 1933 respectively, both killing several times more people than those who died directly from the tremors, Xinhua said.

Underscoring the urgency, Premier Wen Jiabao visited the lake on Thursday, urging workers there to ensure there were no casualties and calling it a "critical moment" for Tangjiashan.

More than 250,000 people have been evacuated in quake-ravaged areas of Beichuan, Mianyang and Jiangyou, adding to the millions already displaced when their homes collapsed or were badly damaged in the earthquake.

Troops went door-to-door in some evacuated towns downstream on Thursday to ensure residents had moved to higher ground.

The more than 600 soldiers who opened the 475-metre long sluice have pulled back from Tangjiashan, but some planned to return on Friday to dig it deeper, state television said.

The lake also threatens a major fuel pipeline and gas operations owned by PetroChina and Sinopec.

An estimated 1,000 to 2,000 children have been orphaned by the quake, an official with the Ministry of Civil Affairs said on Friday, adding parents who lost their offsprings to the disaster would be favoured for adoptions of these orphans.