SHANGHAI, China—The Chinese government announced today that the death toll from Monday's devastating earthquake could climb to more than 50,000 people.

The new estimate came as the country's defense minister gave orders for 101 helicopters to airlift people out of less populated parts of affected areas in Sichuan Province, in an effort to stem the number of casualties caused by the magnitude 7.9 temblor.

Rescue workers, using microphones and fiber-optic cable to probe rubble, frantically are searching for trapped victims. Nearly 20,000 have died to date, with an estimated 40,000 missing. The government's latest figures on the final death toll suggest that many of those yet to be found are likely dead.

Nineteen British tourists reported missing earlier in the week were found and airlifted out of the famed Wolong National Nature Reserve, the British Embassy revealed today; 12 U.S. tourists who were visiting the Wolong Reserve also survived. The reserve—home of the panda breeding program—is located near the epicenter of the quake in Wenchuan County.

The earth shook at 2:28 P.M. local time Monday, causing major damage in the provincial capital of Chengdu as well as the city of Chongqing. Minutes later, Chengdu experienced its own smaller quake—magnitude 3.9—that triggered the evacuation of office buildings.

The earthquake damaged schools, hospitals, roads, railway lines, factories, chemical plants, power lines and dams. Many survivors are sleeping outdoors for fear of aftershocks—a powerful one that measured 6.1 on the Richter scale ripped through the province on Tuesday.

Additional aftershocks have wreaked further havoc, including the collapse of a middle school in the Sichuan city of Dujiangyan that trapped 900 children. Just east of the epicenter, 1,000 students and teachers were reportedly killed or missing at a collapsed high school in Beichuan County. China's Premier Wen Jiabao flew to the province Monday to direct disaster relief efforts and troops were dispatched to the area to help.

This earthquake was the worst to hit China since 1976, when a magnitude 7.5 temblor struck the northern Chinese city of Tangshan, leaving 240,000 dead. Despite decades of effort since then, seismologists still have not settled on a reliable  early warning system for such deadly quakes, though systems are used in Japan and Europe. Nevertheless, Chinese media have included anecdotal reports of unusual swarms of toads in the region immediately prior to the devastating quake.

—From David Biello in Shanghai and wire reports