Beijing  - Troops on Friday rescued at least 17 people who were buried under rubble in the earthquake-devastated province of Sichuan, some survivors escaping after being trapped for more than four days.

President Hu Jintao travelled to Sichuan to rally rescue teams for the "most crucial phase" of relief work in Sichuan, while the confirmed death toll rose to 21,500, one day after the government warned that more than 50,000 could have died in the quake.

Troops in Beichuan town, close to the epicentre, rescued 17 people and reported hearing more "weak yelling for help" from under rubble at the Beichuan middle school, where up to 1,000 people were thought to have been buried.

The troops had rescued another student late Thursday, 80 hours after the quake, the official Xinhua news agency said.

Among those saved in Beichuan on Friday was Feng Shaorong, a man in his 40s who was pulled out of rubble about 97 hours after the quake, the agency said.

In nearby Hanwang town, rescuers also reported signs of life in the rubble, it said.

"The possiblity is very great that we can rescue the buried," it quoted one rescuer as saying in Hanwang.

"Giving-up is excluded from our dictionary," the rescuer said.

Hu's plane landed in Mianyang city, which administers Hanwang.

"We must make every effort, race against time and overcome all difficulties to achieve the final victory of the relief efforts," state broadcaster CCTV and other state media quoted Hu as saying from a plane en route to Sichuan.

Hu said relief work was carried out in a "forceful, orderly and effective" way but added that rescue services still faced a "daunting challenge."

"Saving lives is still the top priority of our work," he said, even though the "golden time" of the first 72 hours after Monday's earthquake had passed.

"We need to make greater efforts in treating the injured, restoring the transportation, telecommunications and power supply infrastructure in quake-stricken areas and ensure basic living conditions for local residents," he said.

Premier Wen Jiabao has stayed in Sichuan since Monday to oversee relief work, flying by helicopter to some of the worst-hit areas on Wednesday and Thursday.

On Friday, Wen said the government would do "whatever the country is capable of to combat the disaster."

The scale of the disaster after Monday's quake, which international seismologists upgraded to 7.9 on the Richter scale, prompted the government to allow foreign rescue services into China for the first time.

A Japanese team of 31 rescuers started work in Sichuan's Guanzhuang town on Friday, where some 700 people were believed to buried under rubble.

A second Japanese team was scheduled to join them in Sichuan, and teams from Russia, South Korea, Singapore and Taiwan were also in or on the way to the province to help with rescue efforts.

Some 20,000 people were evacuated on Thursday as workers reopened two key roads to towns close to the epicentre of the quake.

But damage to several dams and rivers posed new threats to some quake-hit towns and villages, and officials were also concerned about the potential for epidemic diseases to spread in the area.

More than 140,000 troops and paramilitary police were sent to earthquake-hit areas, where several towns close to the epicentre were nearly razed.

Dozens of extra helicopters and planes were brought in on Thursday to drop food, clothing and bedding to residents of Sichuan's worst-hit counties of Beichuan and Wenchuan. (dpa)