BEIJING, China (CNN) -- An unstable lake created by a landslide is China's "most urgent task" in the aftermath of this month's massive earthquake, Vice Premier Hui Liangyu said, according to state-run media.
Tons of earth-moving equipment and explosives have been flown to the site of the quake-created dam in southwestern China's Beichuan county. Engineers are attempting to create a spillway to relieve water pressure as the Jianjiang River fills in behind the massive pile of rock and soil.
"The Tangjiashan quake lake should be our most urgent task," Hui said. "It is threatening millions of lives in the area downstream and any negligence will cause new disasters to people who have already suffered the quake."
Authorities have evacuated 158,000 people from nearly 170 communities, but that number could swell to 1.3 million if engineers become convinced a catastrophic release of water is about to occur, the Xinhua news agency reported.
The so-called quake lake is holding 170 million cubic yards (130 million cubic meters) of water and it has risen to within 83 feet (25) meters of the top of the dam.
The volume of water is equal to about 50,000 Olympic-size swimming pools, according to Liu Ning, chief engineer of the Ministry of Water Resources.
Creating a spillway to relieve the pressure is expected to take 10 days, state media reported, allowing enough time if the lake continues to rise at its average of about 6 feet (2 meters) per day.
Watch the effect of the quake on China's one-child policy
The official death toll from the quake rose to 68,109 on Wednesday, an increase of about 900, with about 19,850 missing. The total number of dead has been increasing daily.
The government estimates that 45 million people, mostly in Sichuan province, were affected by the earthquake and that 5 million were left homeless.
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Earthquake-ravaged Sichuan province suffered through four aftershocks Tuesday, injuring at least 63 people -- six of them critically -- and causing the collapse of more than 420,000 homes, according to Xinhua.
All of them measured between magnitude 4.5 and 5.5, according to the U.S. Geological Survey (USGS). The original earthquake on May 12 registered a magnitude 7.9, the USGS said.
A strong aftershock on Sunday killed at least eight people, injured about 1,000 others and destroyed more than 70,000 homes in Sichuan, Gansu and Shaanxi provinces.
Watch how a survivor reacted to a collapsing mountain »
The worst damage occurred in Sichuan, which has experienced thousands of aftershocks over the past two weeks, but Sunday's -- which the U.S. Geological Survey measured at a magnitude-6.0 -- was the strongest since a magnitude-5.8 tremor shook the region a day after the initial quake.
Shaanxi experienced the highest death toll as a result of the aftershock, with four people losing their lives. One each died in Sichuan and Gansu.
Watch a wedding photographer's shots of the initial quake's impact »
The aftershock damaged more than 200,000 other homes, according to state media. It also damaged another dam, cutting off several more roads in the region.
Meanwhile, Chinese officials on Monday emphasized the country's one-child policy allows families with a child killed, severely injured or disabled to have another baby.
CNN's Yuli Yang and Samson Desta contributed to this report.
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