The devastation was worsened by the blockage of the river upstream, which created a 3km-long lake that overfilled and sent massive waves of mud, rocks and water crashing down into Zhouqu County.
"A preliminary estimate is that 50,000 people have been affected by the disaster," state television said. "About half of the county seat was covered."
State broadcaster CCTV said 45,000 people had been evacuated from the area.
Wen Jiabao, the Chinese premier, visited the area on Monday and promised to help the survivors.
"The key tasks and challenges are expanding the scope for search and rescue, dealing with the barrier lake (caused by mudslides) in a timely and scientific manner, cleaning the sludge and resuming the supply of drinking water," Wen was quoted by the official Xinhua news agency as saying Monday.
He said that relief efforts would continue as long as hopes of survival existed.
Pictures from Zhouqu county showed mud and water in the town streets, and troops frantically digging through debris to hunt for victims, including a boy pulled from a shattered house.
Al Jazeera's Melissa Chan, reporting from Beijing, said "troops have been deployed, the liberation army is on their way and there are some rescue workers".
"The main issue here is not so much the flooding ... but the landslides and basically we have a lot mud," she said.
Xinhua said the mud dumped on the streets was up to one metre deep.
"Now the sludge has become the biggest problem to rescue operations. It's too thick to walk or drive through"
head of Zhouqu county
"Now the sludge has become the biggest problem to rescue operations. It's too thick to walk or drive through," Diemujiangteng, the head of the county, said, according to Xinhua.
Many houses were buried in the landslides which destroyed roads and bridges, while telecommunications, water and electricity have been cut in parts of the region, reports said.
"Someone said the fifth floor of my residential building has been submerged. People are busy looking for their family members and friends," Xinhua quoted Li Tiankui, a local resident, as saying.
Han Jiangping, a merchant in Zhouqu, told the Reuters news agency: "Many single-story homes have been wiped out and now we're waiting to see how many people got out.
"We've had landslides before, but never anything this bad. People are trying to find their families and waiting for more rescuers."
Many residents of Gannan are ethnic Tibetan herders, and the rough terrain may hamper rescue efforts.
The government sent experts on two helicopters to survey the devastation and examine how to blast open the remaining flooded blockage in the valley, according to state television.
Tents, boxed meals and medical supplies were being sent to the area, where more rain was forecast in the coming days.
More than 1,000 people have already been killed this year in floods that have devastated some areas, especially in central and southern China.
Storms in northeast Jilin province killed or left missing more than 100 people in recent days.
The floods have also caused tens of billions of dollars in damage across 28 provinces and regions.
Overall this year, about 875,000 homes have been destroyed, 9.61 million people evacuated, and 22 million acres of crops ruined, according to the government's flood control office.