DUJIANGYAN, China - Bodies covered with sheets lined streets as rescue workers dug through schools and homes turned into rubble by China's worst earthquake in three decades in a desperate attempt to rescue victims trapped beneath concrete slabs. The official death toll rose Tuesday to nearly 12,000, and thousands remained missing.

But hope that many survivors would be found was fleeting. Buildings were knocked down on every block in some cities, and corpses were laid out in the street and in schoolyards.

Only 58 people were extricated from demolished buildings across the quake area so far, China Seismological Bureau spokesman Zhang Hongwei told the official Xinhua.

"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, adding that rescue efforts could take a week.

A day after the powerful 7.9 magnitude quake struck, state media said rescue workers had only just reached the epicentre in Wenchuan county - cut off by the disaster and where the number of casualties was unknown. China said it would welcome international aid but would not yet allow foreign relief workers into the affected area.

Heavy rain, which had contributed to the difficulty of reaching the epicentre, continued to impede efforts and a group of paratroopers called off a mission to the area, Xinhua said.

The death toll rose to 11,921, Wang said. At least 4,800 people remained buried in Mianzhu, 100 kilometres from the epicentre, Xinhua said, citing local authorities.

The casualty figures were expected to rise and remained uncertain due to the remote areas affected by the quake and difficulty in finding buried victims.

The earthquake caused a wide swath of damage across central China, levelling buildings and severing roads and communications. It sent people rushing out of their offices across the country in Beijing, and was felt as far away as Vietnam.

Nearly 10,000 people died in Sichuan province alone and 300 others in other provinces and the mega-city of Chongqing, Xinhua reported.

A 40-car freight train with 13 gasoline tankers derailed in the quake and was still burning Tuesday, the agency said, with no word on casualties.

Earthquake rescue experts in orange jumpsuits extricated bloody survivors on stretchers from demolished buildings, and some 34,000 troops swarmed into the region to help.

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

In Dujiangyan, rescue teams were trying to get to a woman who was eight months pregnant and trapped in a seven-storey apartment building that collapsed.

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."

Nearby, a man in his late 50s who refused to give his name, said his father was missing in the rubble of his home. "Yesterday, when the earthquake happened our home collapsed really quickly and I heard my father yell, 'Help, help, help,"' the man said.

People were seeking rides out of town, where makeshift tent cities were being erected as shelter from rain that began Tuesday and could affect rescue efforts.

Just east of the epicentre, 1,000 students and teachers were killed or missing at a collapsed high school in Beichuan county. The six-or seven-storey building was reduced to a pile of rubble about two yards high, according to Xinhua. Another 900 students were feared dead when their school collapsed in Juyuan, which is in Dujiangyan city.

The Beichuan school had more than 2,000 students and teachers in three school buildings. The other two buildings collapsed partially, Xinhua said.

Up to 5,000 people were killed and 80 per cent of the buildings had collapsed in Beichuan, Xinhua said, in a region of small cities and towns set amid steep hills north of Sichuan's provincial capital of Chengdu. The government has poured more than 16,000 troops into the area with tens of thousands more on the way.

Premier Wen Jiabao, who flew to the area to oversee rescue efforts, said a push was on to clear roads and restore electricity as soon as possible.

China's Ministry of Health issued an appeal for blood donations to help the victims of the quake. "There is a large demand for blood in quake-hit areas and we hope the public actively donate blood," spokesman Mao Quan said.

Before the rescue workers arrived, the only previous contact with hard-hit Wenchuan, Xinhua said, was a satellite phone call from the local Communist party secretary to appeal for air drops of tents, food and medicine.

The official, Wang Bin, said there were 57 reported deaths so far, with more than 300 other people seriously injured. He said the figures were likely to rise as there was no information from mountainous areas.

He estimated that at least 30,000 of the county's 105,000 residents slept outside Monday night.

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

The Chinese government said it would welcome outside aid. Foreign Ministry spokesman Qin Gang said relief authorities "are ready to make contact with relevant countries and organizations."

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.

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