The mud and rock dam -- caused by a landslide on the Jianjiang River -- was also in danger of collapsing under the pressure of the mounting volume of water behind it. Seepage was already occurring, said the spokesman.
More than 600 armed police and soldiers dug a 475-meter channel to divert water from the lake.
It was expected the lake, which holds more than 200 million cubic meters of water, would start draining as soon as the water level reached the lowest point of the dam.
But a headquarters spokesman warned late on Wednesday the chances of bursting were increasing due to uncertainties. These included aftershocks, rain on the upper reaches and the instability of the mud and rock dam.
"Water has been seeping out of the dam body," he said.
A massive landslide could occur at any time on the upper reaches, which could bring about 20 million cubic meters of mud and rock into the lake, triggering huge waves.
"We must prepare for dealing with the worst scenario, but strive for the best results," said Chen Lei, Minister of Water Resources, at a meeting Wednesday in Sichuan.
He said managed drainage was necessary to "ensure the safety of people downstream."
It was not clear when the draining would start.
More than 250,000 people in Mianyang have been relocated under a plan based on the event that a third of the lake water breaches its banks.
Two other plans required the relocation of 1.2 million people if half the lake volume was released or 1.3 million if the barrier fully opened.
The timing of the drainage must be decided by water inflow, said ministry chief engineer Liu Ning.
The catchment area of the upper reaches covered 3,350 sq km. It was estimated that 2 mm of rain in that area would raise the lake level by 1 meter.
According to weather records, average rainfall in the Tangjiashan area in June stands at 140 mm, in July at 360 mm and in August at 330 mm. Rainfall of such levels would pose grave danger.
Another 10,441 residents in a low-lying area about 50 km from the lake were evacuated on Thursday afternoon, and a traffic ban was enforced in downstream areas. More than 250,000 people have been relocated.
Emergency workers were also searching for residents who were missed in the evacuation.
The May 12 quake triggered massive landslides in Sichuan, blocking the flow of rivers and creating more than 30 unstable "quake-formed lakes" that threatened millions of people downstream.
The 8.0-magnitude quake centered on Wenchuan County, about 100 km southwest of Beichuan, has left more than 69,000 people dead, about 18,000 missing and millions homeless. More than 10,000 aftershocks have been reported since May 12.
An aftershock measuring 5.0 on the Richter scale jolted Sichuan's Qingchuan County at 12:41 p.m. on Thursday, according to the China Earthquake Administration.
(Xinhua News Agency June 6, 2008)