(Adds rise in water level)

By Chris Buckley

CHENGDU, June 6 (Reuters) - China readied on Friday to ease pressure on a swelling "quake lake" threatening hundreds of thousands of people in the southwestern province of Sichuan while forecast rains could bring more trouble on the weekend.

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

Water banked up behind Tangjiashan's natural mud-and-rock dam edged to just 23 cm (9 inches) from a sluice built by troops in recent days, state television said, which meant partial discharge of floodwaters building up behind could occur within hours.

But an official from nearby Mianyang city who is a spokesman for the lake relief effort, said that he had no firm idea when the overspill may begin flowing downstream and nor was there any certainty that the larger build-up of water would soon ease.

"We don't know when the sluice will start working. We can only observe and wait," the official, Zhou Hua, told Reuters.

"The sluice will take off the excess water and prevent additional pressure on the dam. But the bigger question is whether there'll be a collapse of part of the dam wall that will release more water. That's why we have these evacuation plans."

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.

The risk of the unstable dam collapsing was increasing due to quake aftershocks and rain, the official Xinhua news agency said.

The State Flood Control and Drought Relief Headquarters warned of heavy rain across China, including Sichuan, over the weekend. Tian Yitang, deputy head of the headquarters, said the rain could pressure quake-damaged waterworks.

"We should be on high alert for flood problems as the previous round of heavy rain had already raised the water levels in some rivers, and irrigation works and reservoirs in quake-hit areas were still not repaired," Tan said, according to Xinhua.

Damaged reservoirs in Sichuan had been ordered to run at a low water level to prepare against possible flooding in the rainy season, according to the provincial bureau of water resources.

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

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.

China's State Administration of Cultural Heritage estimated that about 6 billion yuan ($857 million) will be needed to repair and protect temples, Buddhist rock carvings and other cultural treasures battered by the quake. (Additional reporting by Lindsay Beck and Guo Shipeng; Editing by Jeremy Laurence)

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