Rescuers battled through landslides and clogged roads in a bid to reach victims of twin earthquakes in northwest China Monday which killed 89 people, injured almost 600 and damaged more than 20,000 buildings.

The tremors in Gansu province -- with magnitudes of 5.9 and 5.6 -- triggered landslides which buried often crudely constructed local houses, government-run broadcaster CCTV reported.

Pictures from the scene showed simple buildings reduced to rubble, with pieces of corrugated metal roofing scattered over the wreckage.

The government of Dingxi city in Gansu said on a social media account that 14 people remained missing.

Almost all the confirmed dead were in Min county, where a medicine factory employee told AFP he saw tower blocks shake "ferociously" when the quake struck.

"I was in the workshop. I felt violent shaking and so I ran to the yard of the plant immediately," said the man, surnamed Ma.

"Our factory is only one floor. When I came to the yard, I saw an 18-storey building, the tallest in our county, shaking ferociously, especially the 18th floor."

An official at the provincial earthquake bureau said more than 1,200 buildings collapsed and 21,000 were severely damaged, adding that 371 aftershocks had been recorded.

Zhu Wenqing, a 40-year-old farmer in Min county, told Xinhua news agency he had just got up when his house started shaking.

"I escaped immediately on hearing a 'bang' and feeling the tremor," he said, adding that his house collapsed after several aftershocks.

Nearly 3,000 firefighters, police, soldiers and local government workers had been sent to the area, Xinhua said, but rescue efforts were being hampered by landslides and roads which had been blocked by heavy rain in previous days.

President Xi Jinping ordered an "all-out rescue effort", Xinhua said.

"We are rushing to the scene," Dingxi's vice-mayor told CCTV, which showed an orange-suited rescue worker riding on a tractor.

A red rainstorm alert was issued for the area, which could further obstruct rescue attempts in the mountainous region.

"The damage to houses made from earth bricks has been severe and many are now unusable," the vice-mayor added.

In one location 12 people were buried, the broadcaster quoted a witness as saying. "The rescue work is tough, because the house has been completely buried," the man said.

The civil affairs ministry has allocated 10,000 tents, 30,000 cotton quilts, 5,000 folding beds and 10,000 sleeping bags to Gansu, while the provincial government has set aside 5 million yuan ($830,000) for relief efforts, Xinhua said.

The US Geological Survey said the initial 5.9-magnitude quake hit at 7:45 am (2345 GMT Sunday) at a depth of just 9.8 kilometres.

A second 5.6-magnitude tremor hit the same region at 9:12 am and was 10.1 kilometres deep, USGS said.

While Gansu is one of China's more sparsely populated provinces, Dingxi city, which includes Min and other affected counties, has a population of about 2.7 million.

The quake was felt in the provincial capital Lanzhou and as far away as Xian, the capital of the neighbouring province of Shaanxi, Xinhua reported.

People posting on China's hugely popular microblogs expressed sympathy for the victims. "I hope the dead will rest in peace," read one typical remark.

Beijing's own China Earthquake Networks Centre put the magnitude of the larger quake at 6.6.

The China Earthquake Administration said the same fault zone was linked to a magnitude 8.0 quake on July 21, 1654, it reported.

The USGS rated Monday's main tremor at seven on its "shakemap", with shaking perceived to be "very strong".

Much of western China is prone to earthquakes.

A magnitude 6.6 earthquake in neighbouring Sichuan province killed about 200 people earlier this year, five years after almost 90,000 people were killed by a huge tremor in the same province.