Qinghai's mountainous Yushu county, which was hit by a 6.9 magnitude quake Wednesday, is more than 4,000 metres (13,000 feet) above sea level on the Tibetan plateau, known as the "Roof of the World."

High-altitude rescues were "a more physically and technically demanding task for relief workers due to a shortage of oxygen on the plateau," He Xiong, head of the Beijing Centre for Disease Prevention and Control, was quoted as saying by China Daily.

He was part of emergency operations after the huge quake that struck Sichuan province in May 2008, leaving nearly 87,000 people dead or missing in the worst disaster in China in more than three decades.

Xinhua news agency also quoted Wei Jianmin, a member of a national rescue team sent to Yushu from Beijing, saying that the altitude's impact on his team was one of several factors "adding to the difficult of rescue efforts."

Altitude sickness often occurs when people who are not acclimatized move quickly from lower altitudes to heights of 2,500 metres and above, and symptoms can range from mild to life-threatening.

The oxygen levels in Yushu can be between two-thirds to less than half those found at sea level, the report said, citing government statistics.

Liu Xiangyang, deputy chief of the National Earthquake Disaster Emergency Rescue Team, said his team had worked at Qinghai's high altitudes before.

"We once attended a rescue in 2005 when an avalanche hit Qinghai province. We have some experience in working in high-altitude regions," Liu was quoted as saying.

China's People's Liberation Army, or PLA, has sent 100 paratroopers and 1,500 other personnel to help the rescue operation, China Daily said.

The quake, which hit Wednesday morning, and a series of strong aftershocks have damaged roads and triggered landslides, disrupting telecommunications and knocking out electricity supplies.

Rescue workers were forced to use their bare hands or sticks to dig through the rubble while awaiting the arrival of heavy equipment.

After the 2008 earthquake, high altitude and bad weather meant only 15 of 100 paratroopers reached their targets when they parachuted into Sichuan's mountainous region 5,000 metres (16,400 feet) above sea level, state media previously reported.

The PLA has since been training to improve its long-range airborne deployment after Sichuan earthquake left some troops stranded by rock and mud slides, according to previous reports.

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