Rescuers work in the ruins of a damaged house in Hetuo township in Dingxi, northwest China's Gansu province on July 22, 2013

State broadcaster CCTV showed images of soldiers digging through earth and sand to reach simple houses buried under landslides in the northwestern province of Gansu.

Seriously injured patients wrapped in blankets were put into helicopters heading to the provincial capital Lanzhou, which has the nearest major hospital.

The twin earthquakes that struck on Monday morning had magnitudes of 5.9 and 5.6 according to the US Geological Survey, but were only 10 kilometres (six miles) deep, so that much of the energy released was transmitted to the surface, where it wreaked havoc.

China's official Xinhua news agency said initial investigations showed at least 5,785 houses had collapsed and another 73,000 were severely damaged.

The overnight toll stood at 89 dead, reports said, citing the city government of Dingxi, which includes the worst-affected counties, and Longnan.

Hundreds of aftershocks were recorded in the disaster zone, an area of dusty, jagged mountains.

CCTV showed makeshift tent relief centres being set up, with water, instant noodles and blankets being handed out.

Throughout the night scores of rescue vehicles headed south from Lanzhou to the quake area.

Many rescue workers had travelled from across the country and refused to rest during the night as they raced to find survivors.

"We know the road is more dangerous when it is dark, but we cannot waste a second," said a relief worker who had flown from Beijing with 12 volunteers.

His group joined convoys of army vehicles and ambulances on the highway to Min county, the worst-hit zone.

Many residents in Dingxi town centre felt the main tremor despite the epicentre being some 185 kilometres away.

"I was on the second floor of our building, but it felt very powerful," said the manager of the six-storey Haitian hotel, who is surnamed Xia.

"Many people staying on the higher floors ran out of their rooms, looking rather stunned," she told AFP.

Heavy clouds had gathered over the quake region early Tuesday, as farmers in the largely rural area tended their goats on the mountainsides.

Any storms could hamper the rescue efforts, bringing with them the threat of further landslides.