JIANGYOU, China — A "relatively strong" aftershock Sunday shook the massive earthquake-formed lake that has been threatening to flood more than 1 million people and triggered landslides in surrounding mountains, a state news agency reported.

The effect of the 20-second temblor on the Tangjiashan lake was not immediately known, Xinhua News Agency said. The dam of unstable mud and rocks was under surveillance following the aftershock.

The magnitude of the aftershock was not immediately known, Xinhua said.

Though water has been draining from a hastily dug diversion channel for nearly two days, the lake has continued to swell.

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 a second spillway on the other side of the dam.

The Tangjiashan lake, created when a landslide dammed the Tongkou River, has become a priority for the government as it works 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, have 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 its continuing threat to quake survivors, even before the aftershock. Though water began draining from the diversion channel early Saturday, 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 Sunday evening.