BEICHUAN, China - A strong aftershock sparked landslides near the epicenter of this week's powerful earthquake today, burying vehicles and again cutting off ravaged areas of central China.

Elsewhere, rescuers were still finding survivors after being buried in rubble for 96 hours, while public anger grew over the hundreds of children crushed to death in schools that collapsed in Monday's magnitude 7.9 temblor. More than four days since the disaster, the first foreign rescue workers were allowed to the scene.

An aftershock rattled parts of central Sichuan province this afternoon, the official Xinhua News Agency said, citing its reporters at the scene. A number of vehicles were buried on a road leading to the epicenter, and casualties were unknown.

The U.S. Geological Survey said the latest tremor measured magnitude 5.5 and was centered 6.2 miles below the surface, a relatively shallow quake like the initial disaster.

The government raised the death toll to 21,500 but has said fatalities could rise above 50,000. Tens of thousands could still be buried in collapsed buildings in Sichuan province, where the quake was centered, officials say.

President Hu Jintao made his first trip to the disaster zone, rallying troops among the massive relief operation of some 130,000 soldiers and police.

"The challenge is still severe, the task is still arduous and the time is pressing," Hu was quoted as saying by Xinhua. "Quake relief work has entered into the most crucial phase. We must make every effort, race against time and overcome all difficulties to achieve the final victory of the relief efforts."

A Japanese rescue crew arrived early today -- the first international relief workers in the disaster zone. China initially was reluctant to accept foreign offers of help, but the Foreign Ministry said early today that specialist teams from Russia, South Korea and Singapore also were welcome.

It was the first time ever that China accepted outside professionals for domestic disaster relief, Foreign Ministry counselor Li Wenliang told Xinhua.

Singapore's Foreign Ministry said a 55-member team would arrive in Sichuan later today. Taiwan's Red Cross also has said China agreed to accept a 20-person emergency relief team from the island. Taiwan is sending a cargo plane to Chengdu with tents and medical supplies.

Taiwan and China, which split during civil war in 1949, have banned regular direct links and other formal contacts as political disputes persist.

Chinese government officials, meanwhile, struggled to answer questions from angry citizens on why so many schools collapsed in Monday's earthquake. They vowed to punish anyone responsible for shoddy construction.

In a rare online exchange with ordinary Chinese, the first measure of the number of destroyed schoolrooms -- 6,898 -- emerged, with figures still to come from the hardest-hit areas.

"If quality problems do exist in the school buildings, we will punish those responsible severely and give the public a satisfactory answer," Han Jin, head of the Ministry of Education's development and planning department, said on a state-run forum.

In one case, a high school in Juyuan collapsed in seconds, killing all but a handful of the 900 students. In Mianzhu, close to where Hu arrived, seven fallen schools buried 1,700 people, Xinhua said, and about 1,300 bodies have been recovered so far.

Another 700 students were thought to be buried in a school in Hanwang town. Farther north in Beichuan, about 700 students were also still buried.

A day past what experts call the critical three-day window for finding buried survivors alive, rescuers pulled a nurse to safety who had been trapped for 96 hours in the debris of a clinic in Beichuan county. Two other victims were rescued alive elsewhere in that area.

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