SHANGHAI, China—The death toll from a 7.9-magnitude earthquake that rocked western China yesterday rose to nearly 12,000 and scores more were feared dead as rescuers continued to sift through the rubble of flattened schools and homes in search of thousands still missing, according to Xinhua news agency reports from the local government.

Wang Zhengyao, disaster relief division director at the Ministry of Civil Affairs, said that 11,921 people had died so far in the country's worst earthquake in three decades.

The worst hit areas remain inaccessible to rescuers, due to the remoteness and difficulty of the mountainous terrain of western Sichuan province. The death toll may continue to rise as officials are counting bodies as they discover them--and have recorded deaths in nearby provinces such as Gansu, Shaanxi and Yunnan, according to the Ministry of Civil Affairs.

The quake, which lasted several minutes, has leveled buildings in cities and towns across the province as well as cracking office buildings in the capital city Chengdu where many spent last night out of doors in fear of aftershocks. More than 26,000 have been injured and at least 9,400 remain trapped in such leveled buildings. The hardest hit area is the city of Mianyang, where more than 7,000 have been confirmed killed out of a population of more than 5 million.

Rescuers and troops worked frantically to clear roads turned to rubble by the powerful quake or blocked by falling rocks and mud. Conditions have also been worsened by persistent rain, but soldiers on foot reached the epicenter of the quake earlier today.

More troops and rescuers continued to pour into the region as world leaders offered aid, including the European Commission. And Chinese citizens across the country queued to donate blood for the relief effort.

"It's a nightmare. It's a tragedy," said Zhao Mu, a student at Shanghai International Studies University, echoing the sentiments of the world.

The epicenter of the quake was in Wenchuan County in Sichuan Province near the famed Wolong Nature Reserve, home of the panda breeding program--but was felt as far away as Bangkok.

The earth shook at 2:28 p.m. local time Monday and caused 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.

Aftershocks rattled the western province throughout the afternoon, causing yet more damage, 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 Jibao flew to the province to direct disaster relief efforts and troops were dispatched to the area to help.

According to the U.S. Geological Survey (USGS) official, Monday's tremor was relatively shallow—such quakes tend to do more damage near their epicenters than deeper ones. Another shallow quake, measuring magnitude 7.5, struck the northern Chinese city of Tangshan in 1976, killing more than 250,000 people. That temblor, which was slightly less intense than today's quake, is the second deadliest in recorded history.

Earthquakes are initiated by the release of energy stored in rocks clustered around a fault, which separates masses of Earth's crust known as tectonic plates. These rocks are held in place by friction. As time passes, the movement of the plates causes the rocks around the fault to bend and stretch. This causes the rocks to act like compressed springs, storing energy until the friction across the fault is no longer able to hold the rocks back, and a crack forms across the fault as it begins to slip. This releases part of the built-up energy, some of which creates the seismic waves that travel to the surface and cause damage.

Earthquakes stop when there is not enough energy to keep them going. The energy released by the sliding fault needs to be substantial enough to break the friction holding rocks in place. In some cases, rather than slipping, the two sides along a fault rub together, which can cause a destructive, high-speed quake.

Although the Richter scale has no upper limit, the most devastating earthquakes it measures have magnitudes of 8.0 or higher. More moderate quakes register at 4.5 or greater in magnitude, whereas those of 2.0 or less are typically referred to as "microearthquakes," according to the USGS. American seismologist Charles Richter (1891 to 1989) developed the eponymous scale in 1935 to quantify earthquake magnitude, or strength.

From David Biello in Shanghai and wire reports