Some 87 of the victims were in the city of Dingxi in Minxian County, along the border with Zhangxian County, the official Chinese news agency Xinhua reported.
Authorities say the early-morning tremor injured hundreds and caused major damage.
A preliminary report indicated more than 1,200 houses collapsed and about 21,000 sustained severe damage, provincial government spokesman Chang Zhengguo said during a news conference in the provincial capital of Lanzhou, Xinhua reported.
More than 400 aftershocks were recorded in the quake-hit region, the strongest measuring 5.6 magnitude, Xinhua said.
Zhu Wenqing, a farmer in Meichuan County, told Xinhua his house survived the earthquake but collapsed in one of the aftershocks.
Two helicopters and about 3,000 police officers, firefighters, military personnel and local government workers were dispatched to the region to assist in rescue efforts.
Provincial authorities sent about 500 tents and 2,000 quilts to the quake-stricken area. The Red Cross Society of China sent an additional 200 tents, 1,000 family emergency kits and 2,000 cotton-padded jackets.
Communications and power in portions of the province near Mongolia affected by the quake was down, officials said.
The quake was centered 8 miles east of Chabu and 110 miles south-southeast of Lanzhou.
Residents in Minxian said it lasted about a minute.
The Xinhua report said the region had been recovering from weeks of destructive torrential rains and flooding before the quake struck.
More heavy rain was forecast.
CNN reported some residents in the stricken areas relied on China's Weibo microblog service similar to Twitter to alert the outside world of the disaster.
"This morning at 7:40 I was brushing my teeth, all of a sudden everything shook for a few moments, I thought I didn't get enough sleep last night and was feeling dizzy," one resident wrote. "Turns out it was an earthquake, sigh, seems that with the huge rain downpour outside, we really don't know how much longer this planet is going to let us live here."
Another person said: "Gansu earthquake. So many natural disasters in so short a time, another flood, another landslide, another earthquake, another something. And it's not finished, my God is there any safe place left? Wish everybody a life of peace."
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