DUJIANGYAN, China (AP) - The toll of the dead and missing soared as rescue workers dug through flattened schools and homes on Tuesday in a desperate attempt to find survivors of China's worst earthquake in three decades.

The official Xinhua News Agency said the death toll exceeded 12,000 in Sichuan province alone, and 18,645 were still buried in debris in the city of Mianyang, near the epicenter of Monday's massive, 7.9-magnitude quake.

The Sichuan Daily newspaper reported on its Web site that more than 26,000 people were injured in Mianyang.

The numbers of casualties was expected to rise due to the remoteness of the areas affected by the quake and difficulty in finding buried victims.

There was little prospect that many survivors would be found under the rubble. Only 58 people were extricated from demolished buildings across the quake area so far, China Seismological Bureau spokesman Zhang Hongwei told Xinhua. In one county, 80 percent of the buildings were destroyed.

Rain was impeding efforts and a group of paratroopers called off a rescue mission to the epicenter due to heavy storms, Xinhua reported.

More than two dozen British and American tourists who were thought to be panda-watching in the area also remained missing.

Officials urged the public not to abandon hope.

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

Premier Wen Jiabao, who rushed to the area to oversee rescue efforts, said a push was on to clear roads and restore electricity as soon as possible. His visit to the disaster scene was prominently featured on state TV, a gesture meant to reassure people that the ruling party was doing all it could.

"We will save the people," Wen said through a bullhorn to survivors as he toured the disaster scene, in footage shown on CCTV. "As long as the people are there, factories can be built into even better ones, and so can the towns and counties."

State media said rescue workers had reached the epicenter in Wenchuan county - where the number of casualties was still unknown. The quake was centered just north of the Sichuan provincial capital of Chengdu in central China, tearing into urban areas and mountain villages.

Earthquake rescue experts in orange jumpsuits extricated bloody survivors on stretchers from demolished buildings.

Some 20,000 soldiers and police arrived in the disaster area with 30,000 more on the way by plane, train, trucks and even on foot, the Defense Ministry told Xinhua.

Aftershocks rattled the region for a second day, sending people running into the streets in Chengdu. The U.S. Geological Survey measured the shocks between magnitude 4 and 6, some of the strongest since Monday's quake.

Zhou Chun, a 70-year-old retired mechanic, was leaving Dujiangyan with a soiled light blue blanket draped over his shoulders.

"My wife died in the quake. My house was destroyed," he said. "I am going to Chengdu, but I don't know where I'll live."

Zhou and other survivors were pulling luggage and clutching plastic bags of food amid a steady drizzle and the constant wall of ambulances.

Just east of the epicenter, 1,000 students and teachers were killed or missing at a collapsed high school in Beichuan county - a six-story building reduced to a pile of rubble about two yards high, according to Xinhua. Xinhua said 80 percent of the buildings had collapsed in Beichuan alone.

At another leveled school in Dujiangyan, 900 students were feared dead. As bodies of teenagers were carried out on doors used as makeshift stretchers, relatives lit incense and candles and also set off fireworks to ward away evil spirits.

Elsewhere in Gansu province, a 40-car freight train derailed in the quake that included 13 gasoline tankers was still burning Tuesday, Xinhua said.

Gasoline lines grew in Chengdu and grocery stores shelves were almost empty. The Ministry of Health issued an appeal for blood donations to help the quake victims.

Fifteen missing British tourists were believed to have been in the area at the time of the quake and were "out of reach," Xinhua reported.

They were likely visiting the Wolong Nature Reserve, home to more than 100 giant pandas, whose fate also was not known, Xinhua said, adding that 60 pandas at another breeding center in Chengdu were safe.

Another group of 12 Americans also on panda-watching tour sponsored by the U.S. office of the World Wildlife Fund remained out of contact Tuesday, said Tan Rui, WWF communications officer in China.

Two Chinese-Americans and a Thai tourist also were missing in Sichuan province, the agency said, citing tourism officials.

Expressions of sympathy and offers of help poured in from the United States, Japan and the European Union, among others.

The Dalai Lama, who has been vilified by Chinese authorities who blame him for recent unrest in Tibet, offered prayers for the victims. The epicenter is just south of some Tibetan mountain areas that saw anti-government protests earlier this year.

Beijing Games organizers said the Olympic torch relay will continue as planned through the quake-affected area next month.

The Chinese government said it would welcome outside aid, and Russia was sending a plane with rescuers and supplies, the country's Interfax news agency reported.

But Wang, the disaster relief official, said international aid workers would not be allowed to travel to the affected area.

"We welcome funds and supplies; we can't accommodate personnel at this point," he said.

China's Ministry of Finance said it had allocated $123 million in aid for quake-hit areas.

The quake was China's deadliest since 1976, when 240,000 people were killed in the city of Tangshan, near Beijing in 1976. Financial analysts said the quake would have only a limited impact on the country's booming economy.

Associated Press writers Christopher Bodeen in Juyuan and Audra Ang in Chengdu contributed to this report.