The three geologists who went missing on a field trip to the remote Kekexili nature reserve three weeks ago were still unfound, rescuers in northwest China's Qinghai Province said Saturday.

More than 1,000 rescuers from Qinghai and Shaanxi provinces, Tibet Autonomous Region, Xinjiang Uygur Autonomous Region and Beijing have been searching the vast expanse of uninhabited land for the missing people and their vehicle, but of no avail.

The three geologists, including 36-year-old Yang Nengchang, 53-year-old Gao Chongmin and 23-year-old Rong Hao, were reported missing on February 19, the day they were scheduled to return to their campsite from a three-day excursion, said Bai Linke, head of the emergency rescue taskforce.

All three worked for a subsidiary of the state-owned Shaanxi Geological and Mineral Resources Development Corporation of northwest China's Shaanxi Province, said Shen Anbin, a company executive who is assisting the rescue efforts.

The geologists arrived in Kekexili last September for a survey that was scheduled to last until the end of April.

On February 17, the three men left on a Cherokee jeep to collect geological data at 60 different locations near Tripug Dratso, a lake area with a large expanse of wetlands and swamps, Shen said.

Their colleagues and relatives have not heard from them since, though they were known to have a satellite phone.

They also had a radio with a communication range of 15 kilometers and two GPS systems. Their food and water supplies, however, could last only seven to 10 days, said Shen.

The lake area, at an average altitude of 5,100 meters, is known for its tough climate and complicated topography.

Bai said they had searched almost every inch of land along the missing geologists' scheduled route, and captured satellite images of the area. "We found a few suspected 'targets' after a thorough analysis of the satellite data, and have sent rescuers to the sites for closer checks."

He said the next phase of the rescue work would focus on the blind areas, including every pit and ravine along the route.

"We'll do our best to search for them, but we also have to protect the rescuers' safety," he said.

Many rescuers are fighting altitude diseases, with three to four falling ill every day, he said.

Kekexili, also known as Hoh Xil, is a reserve that encompasses China's largest area of uninhabited land. It is home to several species of endangered wildlife, including Tibetan antelopes.

The region became well-known following the release of the award-winning feature film, "Kekexili Mountain Patrol" by Chinese director Lu Chuan. It tells the true story of a journalist who joined a Tibetan volunteer patrol to pursue poachers trading Tibetan antelope skins in Hoh Xil.