A magnitude 5.0 aftershock Sunday set off new landslides in the surrounding mountains, underscoring the threat of flooding. Another aftershock of the same magnitude struck Monday afternoon, shaking the Sichuan provincial capital of Chengdu and the hard-hit city of Mianyang for about 10 seconds.
The second temblor sent people in the hard-hit county of Wenchuan rushing out of shaking buildings and tents, Xinhua said. There were no immediate reports of damage or casualties.
Xinhua said experts at the scene said the dam was safe after the aftershock, but authorities remained on alert both for increased rainfall and new aftershocks that could still weaken the dam or send more debris plunging into the lake.
Tangjiashan lake was formed when rubble from a massive landslide set off by the May 12 quake blocked the flow of the Tongkou River, also known as the Jianjiang.
Managing the lake has become a priority for a government working to head off another catastrophe even as it cares for millions left homeless from the quake centered on the 7.9 magnitude quake. More than 1.3 million people live downriver from Tangjiashan.
The death toll from the quake climbed Monday to 69,142, with 17,551 people still missing.
Wooden houses, boulders and other debris have been blasted to speed the flow of water into the spillway. Other People's Liberation Army troops have been deepening the channel and digging on a second spillway.
Television and official Web sites showed troops firing 82-millimeter recoilless guns at debris on Sunday. The troops dislodged enough debris to speed the drainage, but the level continued to rise with the inflow from the blocked river behind the dam, Xinhua reported.
An additional 120 troops were sent to the lake Monday to continue the draining operation.
David Petley, a geography professor at Britain's University of Durham, said the situation at the lake appeared to be reaching a crisis level. "The teams on the dam are fighting a desperate battle now," he said. "The outcome is very uncertain."
The rising water level indicates the outflow was not fast enough, Petley said. From news photos, he said, there are indications that the top of the dam was holding, instead of eroding slowly as it should, while the channel farther down was eroding too quickly.
That could potentially place increased pressure on the dam by suddenly sucking down large volumes of water, overwhelming the barrier, Petley said.
More than 250,000 people downstream have been evacuated in recent weeks, adding to the turmoil created by last month's massive earthquake in China's Sichuan province. Many were living in improvised camps on surrounding hillsides, surviving on instant noodles and suffering from heat, mosquitoes and a lack of water for bathing.
The Tangjiashan lake is the largest of more than 30 created by last month's quake. Government experts quoted by state media have played down the threat of imminent flooding, though a variety of factors could set off a dam collapse: rain, aftershocks, landslides or increased leakage from the barrier.
