CCTV reported that all roads had been cleared by last night, allowing additional rescuers to join those already searching for survivors. Yiliang county in Yunnan and Weining county in Guizhou suffered the most damage.

More than 200,000 people had been relocated, with more than 6,600 houses destroyed and 430,000 damaged, Xinhua said.

Wen flew to Yunnan from Beijing on Friday afternoon and reached the quake zone at nightfall, accompanied by provincial party secretary Qing Guangrong and Governor Li Jiheng, CCTV said.

The 70-year-old premier greeted relief workers, spoke with survivors and visited children injured in the quake before returning to Beijing yesterday.

Three children died when the Fada Centre Primary School in Yunnan's Juekui township caved in with 22 pupils inside, the website of Yunnan's said.

While 15 children managed to escape from the building, seven were buried beneath the rubble. Villagers helped the school's only teacher, Zhu Yinquan, dig out four survivors.

The incident, while isolated, may renew concerns about the problem of poor-quality school buildings, such as those that fell during the magnitude-8 quake four years ago in neighbouring Sichuan province, killing thousands of schoolchildren. So far the Juekui school was the only one known to have collapsed.

Wen Shanwei, a teacher in another primary school in Niujie town, Yiliang county, described the rush of fear he felt as the quake struck his school - assessed as shoddily built after the 2008 quake.

"I was scared when I felt the shaking in a class of 44 pupils on the second floor," Wen said.

More than 300 pupils and nearly 20 teachers managed to escape to safety in the playground within minutes, he said, and no one was hurt.

Meanwhile, the Civil Aviation Administration issued a notice ordering airlines to ensure there were sufficient flights to transport relief materials and personnel to the quake zone.