The quakes struck near the border of Yunnan and Guizhou provinces, the first at 11.19am local time and the second about 45 minutes later, the US Geological Survey said.
About 700 people were injured and 20,000 homes damaged in the remote mountainous region about 210 miles from the Yunnan provincial capital Kunming.
As the number of dead climbed throughout the day, state media reported Premier Wen Jiabao would travel to the area, as he has often done when disasters strike Chinese regions.
President Hu Jintao called for disaster relief to be dispatched to the area while attending the Asia-Pacific Economic Co-operation forum in the Russian city of Vladivostok.
Most of the victims were from Yiliang county in Yunnan, near the epicentre of the quakes, which struck at a depth of about 5.6 miles (9km).
By mid-afternoon, authorities had moved more than 100,000 from the area as a series of more than 60 aftershocks struck. No deaths were reported in Guizhou province.
Calls to police stations and hospitals in Yiliang went unanswered, but a worker at No 2 Renmin Hospital in Zhaotong city said medical staff were busy treating the injured.
"We have admitted injured people, but don't have an overall number yet, and we can't comment without government approval," he said, declining to give his name.
Buildings in China's less developed regions are often thrown up with little regard for construction standards, making them susceptible to earthquakes. CCTV footage showed boulder-covered roadways, abandoned cars and black smoke pouring from buildings.
"The hardest part of the rescue now is traffic. Roads are blocked and rescuers have to climb the mountains to reach hard-hit villages," said Li Fuchun, an official from Luozehe, the town at the epicentre of the quake.
The death toll may rise as rescuers reach villages cut off by landslides.
Many structures in the area are built with mud and timber, making them more prone to collapse, the International Federation of Red Cross and Red Crescent societies said.
However, they said that extricating people trapped in these structures may be easier than from under concrete or brick homes, meaning "there could be many more injuries proportionate to the number of deaths".
In 2008, some 87,600 people were killed in the south-western province of Sichuan when a 7.8 magnitude earthquake hit.
Many of the victims died in the rubble of homes and schools built without adequate steel reinforcement.
A 6.9-magnitude earthquake in April 2010 killed nearly 3000 people in a remote part of western Qinghai province, devastating much of Yushu county, where many displaced by the disaster still live in tents.
Quakes which have an epicentre of less than 70km below the surface are considered shallow and can cause significant damage, even at lower magnitudes.
Christchurch, the largest city in New Zealand's South Island, is still recovering from a 5km-deep quake measuring 6.3 which killed 182 people in February 2011.
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