Tian Baozhong, the director of Gansu's civil affairs department, painted a grim picture, telling reporters that the death toll had more than doubled overnight while the number of missing had dropped only slightly to 1,042.
The mudslides levelled an area five kilometres long and 500m wide, Xinhua said.
Floodwaters up to three storeys high have submerged half the county, where one-third of the population is Tibetan.
With rain forecast for the rest of the week, China's premier Wen Jiabao urged the thousands of rescuers in hard-hit city of Zhouqu to hurry, but acknowledged the task would be difficult.
"We must fully realise the difficulties for the search and rescue work," Wen was quoted as saying by Xinhua.
"You must race against the clock and spare no efforts in saving lives."
|Fears are growing of an outbreak of disease as rescue efforts continue [AFP]
More than 7,000 soldiers and rescuers worked around-the-clock for survivors in Zhouqu, the county seat, where homes were torn apart and streets buried in mud as deep as two metres.
According to Chinese authorities, the landslides swept mud, houses, cars and other debris into the Bailong river running through Zhouqu, choking off the waterway and triggering flooding in the mountainous area.
The Bailong remained flooded on Tuesday, with only the tops of street lamps visible above the water line as Zhouqu residents queued for food and bottled water.
Six specialists in epidemic prevention were sent to the area amid fears that contaminated water could spark an outbreak of disease, Xinhua reported.
Before the Gansu musslides, the government said more than 2,100 people were dead or missing nationwide in flood-related disasters.
More than 12 million people have been evacuated from their homes in the worst flooding in a decade.
A separate landslide on Monday night in Shaanxi province, also in the northwest, killed five people and injured eight others.