Landslides and floods triggered by torrential rain have engulfed a town in north-western China, killing at least 127 people and leaving 1,300 missing.
Nearly 3,000 soldiers and 100 medics have been sent to assist local rescue teams in Zhouqu, in an isolated, mainly Tibetan region of Gansu province.
More than 45,000 people have reportedly already been evacuated from the area.
Local officials say thick mud, more than 1m (3.3ft) deep in some places, is hampering rescue efforts.
Prime Minister Wen Jiabao flew to Gansu after receiving news of the disaster. He told officials to "spare no efforts to save lives".
More than 1,400 people are believed to have been killed this year in unusually severe flooding in central and southern China.
About 1.4m homes have been destroyed, 12 million people evacuated, and 87,600 sq km (22m acres) of crops ruined, officials say.
The landslides happened late on Saturday following torrential rain in Zhouqu county, part of Gannan Tibetan Autonomous Prefecture, officials told the official Xinhua news agency.
Rock and mud slipped down several hillsides and engulfed the town of Zhouqu, which lies in a narrow valley cut by the Bailong river.
"There was thunder and huge rain, and then the landslides started coming down," a resident of Zhouqu told the Reuters news agency.
"That was about midnight, so some people must have been in their homes, asleep and didn't know what was happening."
The landslides swept mud and other debris into the river, blocking it upstream of the town.
Water built up behind the temporary dam, creating a 3km (2-mile) long lake that overflowed and sent waves of water, mud and rocks crashing down on the town.
The deluge flattened houses and tore several blocks of flats in half. One resident said the fifth floor of his building had been submerged at one point. A small hydroelectric power station was destroyed.
Pictures from Zhouqu showed scenes of devastation, with mud, water and wrecked cars covering the town as people searched for missing relatives. State media reported that one village of 300 homes was "buried".
"Many single-storey homes have been wiped out and now we're waiting to see how many people got out," Han Jiangping, a local merchant, told Reuters.
"We've had landslides before, but never anything this bad. People are trying to find their families and waiting for more rescuers."
Officials warned that the death toll could rise, although the number of people missing had been lowered from 2,000 by Sunday evening. Some 680 people had been rescued by midday.
"It's very hard to locate the people washed away by floods. It's hard to say what their chances of survival are," He Youxin, a People's Armed Police officer organising rescue efforts, told Xinhua.
Officials said the thick mud and wreckage made it impossible to use heavy machinery, forcing rescuers to use only spades and their hands.
"Now the sludge has become the biggest problem to rescue operations. It's too thick to walk or drive through," said the head of Zhouqu county.
Tents, food and medical supplies are being rushed to the area, but the remote, mountainous location is making access difficult.