JIANGYOU, China — An aftershock shook a massive quake-formed lake Sunday that had been threatening to flood more than 1 million people, sending landslides tumbling down surrounding mountains, China's state news agency reported.

No evacuations were ordered and the dam did not collapse, said Hu Peng, a media officer at the disaster relief headquarters of the Tangjiashan "quake lake." He had no additional details.

The effect of the 20-second temblor on Tangjiashan was not clear and the dam of unstable mud and rocks was under surveillance, said Xinhua News Agency, which had a reporter at the site. The aftershock had a magnitude of 5.0, according to the U.S. Geological Survey.

One evacuee on a mountainside about 25 miles downstream said the situation remained stable following the aftershock.

"We felt the aftershock but it wasn't anything bad. We're numb to this by now," said Yu Taichun, a doctor living in a tent city on Taohua Mountain.

Authorities had been draining the Tangjiashan lake since early Saturday, using a hastily dug diversion channel to relieve pressure on dam. The lake has been swelling since the deadly May 12 earthquake centered in Sichuan province sent a landslide tumbling into the Tongkou River, blocking its flow.

However, water levels in the lake continued to rise even as water gushed down the diversion channel. Soldiers blew up wooden houses, boulders and other debris Sunday to speed the flow of water into the spillway. Other troops were deepening the channel and digging on a second spillway on the other side of the dam.

The first sluice appeared to be operating smoothly after the aftershock and an accompanying rainstorm, Xinhua reported.

Draining the Tangjiashan lake has become a priority for a government working to head off another catastrophe even as it cares for millions left homeless from the May 12 quake that killed nearly 70,000 people.

More than 1.3 million people live downriver from Tangjiashan, and 250,000 of them have been evacuated.

Government experts, quoted by state media, had played down the threat of imminent flooding, saying Tangjiashan's landslide-created dam should hold. But state media and officials estimated it would be a week before the evacuees could return home, even if all goes well.

The rising water levels of the Tangjiashan lake underscored the persisting threat to quake survivors, even before the aftershock. Though water had been draining for nearly two days, the effect was hardly noticeable in some downstream communities.

The turquoise waters of the Tongkou flowed placidly past the village of Jiuling, about 28 miles downstream.

"I wish they'd hurry, look at us here," said rice farmer Cai Yuhua, gesturing at a cluster of mostly homemade tents built on a nearby hillside, where she and hundreds of others waited out the flooding threat.

"The last time we could go back to our homes was May 22. I want to go home and look at my things," said Cai, who was living under a striped plastic tarp cast over bamboo poles.

The official death toll from the quake crept up Sunday to 69,136 people, with 17,686 still missing.

Meanwhile, a cargo train derailed in northeastern Sichuan province early Sunday after being struck by rocks falling from a mountain, Xinhua said. One railway worker was killed and another was seriously injured.

It was not known if the falling rocks were related to the May 12 quake or its aftershocks. The rocks may have been loosened by recent heavy rains, Xinhua reported, citing a Chengdu railway administration official. The damaged line was repaired by Sunday evening.