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China has sent thousands of rescue workers to help search for survivors of deadly landslides that swept through a county in the northwest of the country.

More than 4,500 soldiers, firefighters and medical staff have been sent to Zhouqu in Gansu province, as well as helicopters and aircraft.

Chinese Premier Wen Jiabao has visited the county, where 127 people are known to have died.

Almost 1,300 other people are missing in the wake of Saturday's landslides.

The landslides were caused by torrential rain, state-run Xinhua news agency reported.

Zhouqu county, which is part of Gannan Tibetan Autonomous Prefecture, lies in a narrow valley cut by the Bailong river.

Water built up behind a landslide in the river, which then burst its banks, sending water, rocks and mud down several hillsides to residential areas below.

"There was thunder and huge rain, and then the landslides started coming down," a Zhouqu resident told Reuters news agency by telephone.

"That was about midnight, so some people must have been in their homes, asleep and didn't know what was happening."

The deluge flattened houses and tore several blocks of flats in half. Images from the area showed streets filled with rocks and mud, overturned cars and buildings with several storeys still under water.

In one village alone, 300 homes had been buried, Xinhua said.

Mr Wen, who flew to the area, called for a swift search effort.

"For those who were buried under the debris, now it's the most crucial time to save their lives," Xinhua news agency quoted him as saying.

Troops with explosives were said to be flying in to tackle the blockage in the river, behind which a lake had built up.

But officials said the thick sludge blocking roads posed the biggest problem to rescue operations, because they could not bring heavy machinery in.

Forty-five thousand people are reported to have been evacuated from the region and almost 700 people rescued from affected areas.

But more rain is forecast for Wednesday and landslides were continuing, Chinese media reports said.