Hu flew to central China to view the devastation and the massive relief operation that included more than 130,000 troops. Four days after the quake hit, soldiers and police finally reached all of the hardest-hit isolated mountain counties and townships at the epicenter, state media said.
A harsh setback came Friday as state media reported strong aftershocks had caused landslides, blocking some of the few usable roads in Sichuan province and burying a number of emergency and recovery vehicles in the process.
And as officials raised the official death toll to about 21,500 in Sichuan, public criticism grew over the many children killed while at school during the magnitude 7.9 quake.
Education and housing officials took the rare move of taking questions online from angry citizens. The government said it would investigate why so many school buildings collapsed - about 6,900 classrooms were destroyed, not including the hardest-hit counties - and severely punish anyone responsible for shoddy construction in the buildings.
Sichuan Vice Governor Li Chengyun announced the new death toll number at a news conference Friday in the provincial capital Chengdu. It was raised by about 3,000 from a day earlier.
A day past what experts call the critical three-day window for finding buried survivors alive, stories of rare survivors being discovered did deep hope alive for rescue workers.
A Chinese state news agency said Friday rescuers had pulled a nurse to safety after being trapped for 96 hours in the debris of a clinic. Xinhua News Agency said two other people were pulled out of a collapsed building in Beichuan in northern Sichuan province an hour earlier.
The area was one of the last that rescuers reached after Monday's earthquake.
Rrescuers pulled a student trapped for 80 hours from a school in the Beichuan area. They said they could hear more weak cries for help.
The Earthquake and Disaster Relief Headquarters of the State Council, the country's Cabinet, 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, the vice governor told reporters.
"The challenge is still severe, the task is still arduous and the time is pressing," the official Xinhua News Agency quoted Hu as saying. "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."
In Mianzhu, close to where Hu arrived, seven fallen schools buried 1,700 people, Xinhua said, with about 1,300 bodies recovered so far.
Weak signs of life were detected in five places amid the rubble of Dongqi Middle School in Mianzhu, where about 100 students were missing, Xinhua reported.
In the same area, 700 students were thought to have been buried in a school in Hanwang town. Further north in Beichuan, 360 students were rescued from the ruins of a school, but another 700 were still buried.
Police restricted the last couple miles of road into Beichuan to emergency vehicles. Military trucks and cranes edged around huge fallen boulders.
CBS News reporter Celia Hatton said thousands of people were trying to make their way back into the devastated rural areas from Sichuan's capital city Chengdu, either to search for loved ones, or just to try and help the rescue effort.
The flow of private vehicles was worrying authorities, who set up checkpoints like the one on the way into Beichuan to prevent traffic from impeding the official rescue effort.
Dozens of people trudged up the winding mountain road, carrying backpacks and bags with food and medical supplies, on a quest for missing relatives.
Liu Jingyong, a 43-year-old migrant worker searching for his cousin, traveled two days by bus and now foot just to get near his relative's home.
"I have not had any information from him," Liu said. "This is so hard on me."
China's Ministry of Land and Resources warned that heavy rains forecast for the next few days would likely set off new landslides.
The ministry said on its Web site that local authorities "must immediately mark off danger zones" to ensure rescue workers' safety. Landslides already have blocked not only roads but also rivers, which have formed new lakes.
The National Development and Reform Commission made an emergency allotment of 53 million yuan ($7.6 million) to repair reservoirs in the region, the agency said on its Web site.
At least one dam was reported to have cracked during the quake, raising fears for communities downriver. But there was no repair work or extra security seen Thursday at the Zipingpu dam, indicating the threat likely had passed.
The government said it had allocated a total 5.4 billion yuan ($772 million) for earthquake relief, according to the central bank's Web site, up sharply from 1.11 billion yuan ($159 million) two days ago.
Given the widespread destruction, AIR Worldwide - a catastrophe risk modeling firm - estimated losses to both insured and uninsured property would likely exceed $20 billion.
The first international relief workers started working in the disaster zone, with a Japanese rescue crew arriving early Friday. China initially was reluctant to accept foreign offers of help, but the Foreign Ministry said early Friday that specialist teams from Russia, South Korea and Singapore were also welcome.
Singapore's Foreign Ministry said a 55-member team would arrive in Sichuan later Friday.
Experts said the time for rescues was growing short.
"Anyone buried in an earthquake can survive without water and food for three days," said Gu Linsheng, a researcher with Tsinghua University's Emergency Management Research Center. "After that, it's usually a miracle for anyone to survive."
Nearly 70,000 injured people were being treated at hospitals in Sichuan, Dr. Shen Ji, director general of the provincial health department, told reporters in Chengdu, adding that no quake-related epidemics have broken out yet.
The Health Ministry posted a notice on its Web site for disease prevention, saying saving lives was the top priority for now, then "public sanitation and epidemic control and prevention should be thoroughly carried out." Bodies should be cleaned and buried as soon as possible, the ministry advised.