Chinese soldiers are hiking across washed-out roads and mud-blocked passageways in a desperate effort to get to survivors in the worst hit areas of this week's earthquake.

In areas of Sichuan province where help has already arrived, rescue crews continued to rummage through piles of debris looking for signs of survivors.

In the largest city near the epicenter, Chengdu, a teacher described how the quake sent rocks crashing down onto her school, crushing students.

"The whole mountain seemed to come down on us," she told CTV's Steve Chao. "We were buried underneath."

At the city's Juyuan Middle School, where 900 of the area's brightest children studied, only one child has been pulled out alive.

The death toll following the worst earthquake to hit China in three decades has passed 12,000 -- but rescue teams were searching for thousands more still buried under crushed buildings.

Fifty-thousand police officers and soldiers have been mobilized to sift through piles of concrete.

The powerful 7.9-magnitude earthquake struck central China Monday at 2:28 p.m. local time, when office buildings, factories and schools were full.

In Mianyang, another city near the earthquake's epicentre, 18,645 people remain buried in debris, the official news agency Xinhua reported.

According to the Sichuan Daily newspaper, more than 26,000 people were injured in Mianyang.

On Tuesday, rescue workers continued to dig through flattened schools and homes, searching for survivors.

"Survivors can hold on for some time. Now it's not time to give up,'' said Wang Zhenyao, disaster relief division director at the Ministry of Civil Affairs.

Wang said rescue efforts could take a week.

But even when survivors are found, crews have had to make heartbreaking choices. Rescuers had to amputate the legs of one little boy who was found in the rubble.

Even those who survived the quake are struggling to survive as they run out of food and try to build makeshift shelters.

Bad weather is hindering help from arriving at the worst hit areas and heavy rain is slowing down relief efforts.

Red Cross delivering aid

Francis Markus, with the International Committee of the Red Cross in Beijing, said Tuesday his organization has been trying to get relief supplies into the devastated areas.

"Clearly, the situation is going to get even more serious, the numbers are likely to rise even further," Markus told CTV's Canada AM from Beijing.

So far, only 58 people have been extricated from demolished buildings across the quake area, China Seismological Bureau spokesman Zhang Hongwei told Xinhua Tuesday.

East of the epicentre, about 1,000 students and teachers were killed or missing in Beichuan county after a high school collapsed Monday.

Another 900 students are feared dead in Juyuan, which is in Dujiangyan city, after their school collapsed.

Aftershocks also rattled the region Tuesday, creating a panic in the city of Chengdu. The U.S. Geological Survey measured the shocks between magnitude four and six.

A spokesperson for Canada's Foreign Affairs said no Canadian casualties have been reported. A B.C. professor and his students were on a study trip to China when they were caught in the earthquake zone. They say they feel lucky to have survived without injuries.

"The overriding sort of emotional sentiment amongst us now is just the feeling that we are really like lucky -- considering what we are hearing now about what is happening in areas that are really close to here," Chris Bertram, a professor at the University of the Fraser Valley, told CTV News.

The number of Canadians registered as living in China is 6,449.

In 1976, 240,000 people were killed in the city of Tangshan following an earthquake.

With a report by CTV's Steve Chao in Chengdu and files from The Associated Press