Freezing weather, high altitude and frequent aftershocks made rescue efforts difficult to reach survivors of a strong earthquake that has left 617 dead, 9,110 injured and 313 missing in northwest China

Rescuers fought altitude sickness, chilly weather, strong winds and frequent aftershocks Thursday to dig through rubble and reach survivors of a strong earthquake that has left 617 dead, 9,110 injured and 313 missing in northwest China.

The 7.1—magnitude quake, which struck the Tibetan Autonomous Prefecture of Yushu in southern Qinghai Province early on Wednesday, also toppled 15,000 residential buildings and forced 100,000 residents to be relocated, according to the rescue headquarters.

Many people are still buried under the debris of collapsed houses in the Gyegu Town near the epicenter, the seat of the Yushu prefecture government and home to 100,000 people. It sits at about 4,000 meters above sea level.

More than 85 percent of houses in Gyegu, mostly made of mud and wood, had collapsed.

“Freezing weather, high altitude and thin air have all made rescue efforts difficult,” said Hou Shike, deputy head of the China International Search and Rescue Team.

Some of the team’s rescuers were already feeling dizzy due to low oxygen level in the air soon after arriving at the Yushu Airport Wednesday night, Hou said.

Roads linking the airport and Gyegu were blocked by landslides triggered by the quake, which hampered rescue missions, he said.

“We have begun searching and rescue operation in Gyegu overnight, and will set up a field hospital today,” he said.

At the Yushu Vocational School, more than 20 bodies have been recovered, but at least 20 others are still buried in the debris.

“Perhaps because of altitude sickness, sniffer dogs did not even work sometimes,” said Chang Zhiqiang, a school official who joined the rescue.

Most of the survivors have to stay in the open area Wednesday night amid freezing weather or sought temporary shelters in buildings that remained unaffected by the tremor. Rescuers have set up more than 40 tents for survivors in Gyegu, but the effort seems far from enough.

Professional rescuers, soldiers, police officers and medical workers have been dispatched to Yushu from across the country, along with tents, cotton—padded clothes, quilts, food, water, medicine, bulldozers, excavators, cranes, generators and other relief supplies.

Donations from governmental agencies, privately—run companies or individuals are also being rushed to the quake—hit region.