Beijing - Chinese Premier Wen Jiabao on Sunday toured an area in northwest China wracked by an avalanche of rain and mud that killed at least 127 people and left hundreds missing - the latest disaster in a summer season that has brought the nation's worst flooding in a decade.

Wen's visit came as rescue teams franticly searched flooded homes for survivors in rain-swollen Zhouqu County in the nation's northwestern Gansu Province. Authorities were reportedly trying to find an estimated 1,300 people, down from an earlier estimate of about 2,000.

A torrential downpour began late Saturday in the province's Gannan Tibetan Autonomous Prefecture, an area dominated by steep and barren terrain. Most of the area's 135,000 residents are ethnic Tibetan herders and farmers.

Shortly after midnight, as residents slept, mountains of mud struck the area, smashing a small power station and wreaking havoc as the Bailong River overflowed its banks.

Half the county was left underwater and one village of 300 homes was literally "buried" under mud and debris, according to the Xinhua news agency.

Peng Wei, head of the county's fire department, told reporters that "the county was in a valley and the river runs in the middle."

State-run television Sunday estimated that 50,000 people had been evacuated. Nearly 3,000 troops and 100 medical workers were converging on the scene, while helicopters surveyed the damage, officials said.

"Now the sludge has become the biggest problem to rescue operations," one local official told Xinhua. "It's too thick to walk or drive through."

Li Tiankui, a resident who lives near the Bailong River, also told the news agency: "Someone said the fifth floor of my residential building had been submerged. People are busy looking for family members and friends."

So far this year, 1,100 people have been killed and more than 600 have gone missing amid floods that have caused tens of billions of dollars in damage across 28 provinces and regions. Overall, some 875,000 homes have been destroyed with more than 9 million residents evacuated.

Record-high rains have raised rivers in numerous regions to perilously high levels, including the fast-flowing Yangtze River. Floodwaters filled the reservoir at the massive Three Gorges Dam to dangerous levels, officials said.

Elsewhere, some 40 large and medium-sized reservoirs in northeast China were near capacity. Officials are investigating the breaching of a reservoir that killed 40 residents in five downstream communities.

On Sunday, state-run TV showed images from Zhouqu County, where cavalcades of mud several-feet deep flowed down the town's streets, sweeping along cars, uprooting trees and other debris. The coverage showed one boy being pulled from a ruined house.

Terrified residents fled to high ground or upper stories of apartment buildings. Power, water and communications were cut in flooded areas.