Update: The death toll from the massive earthquake in central China exceeded 12,000 Tuesday, as rescue workers frantically tried to pull victims from the rubble and clear roads of debris so more relief can be delivered.
A senior official with the Sichuan Provincial government said Tuesday the death toll in the province has surpassed 12,000 and is rising, state-run news agency Xinhua reported.
Li Chengyun, vice governor of Sichuan, said at a news conference Tuesday afternoon that the death toll was based on incomplete figures tallied by 4 p.m. (0900 GMT, 0400 ET) Tuesday.
He said another 26,206 people were injured and as many as 3.5 million homes have been destroyed.
Xinhua also reported that more than 18,645 people were buried under rubble in the city of Mianyang alone -- 3,629 people were also reported dead in the city, which neighbors the epicenter of the earthquake.
A string of nearly 30 seismic jolts hit the province in the first 24 hours following Monday's quake and slowed the progress of 1,300-strong rescue teams. All of those quakes were magnitude 4.0 and above.
A Chinese Civil Affairs Ministry official said his country welcomed foreign donations of money and materials, but it was not ready for outside teams of rescue and relief workers because its transportation system could not handle the additional traffic.
The Chinese government on Tuesday accepted an American offer of $500,000 in relief funds, according to an official with the U.S. Agency for International Development. However, China has not asked for other disaster assistance.
Roads blocked by rocks and mudslides had hampered the effort to reach the epicenter in Wenchuan County, forcing military doctors and soldiers to walk to reach the area almost 24 hours after the 7.9 magnitude earthquake shook central China, Xinhua said.
Chinese prime minister Wen Jiabao ordered the military to make it a priority to open the roads into Wenchuan County, home to about 100,000 people, by midday Tuesday. He arrived in the earthquake zone Monday to personally direct the relief efforts.
"At present, we have great difficulties to carry out our rescue work," Wen said. "Blocked roads, disrupted communication and continuous rainfall have all created obstacles to our rescue efforts."
Several thousand additional soldiers should reach the area later Tuesday afternoon, Xinhua said.
Heavy rains forced the military to cancel plans to drop Chinese People's Liberation Army paratroopers into the Wenchuan area, Xinhua said. Bad weather also has grounded all helicopter relief efforts, the military said.
CNN's John Vause saw block after block of devastation in the town of Jiang You, about 60 miles (100 km) from the epicenter, arriving there about a day after the quake hit.
"These people who live in the city are now hunkering down under tarpaulins and under tents," Vause said, as a steady drizzle added to the misery. "Many are afraid to go back indoors because their buildings are no longer safe."
Communications with survivors near the epicenter has been difficult because of broken telecommunication lines and poor weather. An official using a satellite phone did give an initial report that about a third of all buildings had collapsed and another third were seriously damaged, Xinhua said.
In Guixi Township -- 35 km (22 miles) from the epicenter -- thousands of residents huddled under makeshift tents and tarps, their only shelter from a steady rain Tuesday.
Row after row of houses collapsed during the earthquake, leaving people with no place to go. Many injured and hungry people wandered the streets, creating a scene of human misery. The roads to the town were open, but still no relief workers were around.
An expert told CNN the earthquake, which struck at 2:28 p.m. (0728 GMT, 0228 ET) Monday, was the largest the region has seen "for over a generation."
The area is also the refuge for much of China's panda population. The fate of the 130 pandas housed at the Wolong Giant Panda Protection and Research Center was unknown, Xinhua reported.
Some 20,000 Chinese troops have been deployed to the region, while another 24,000 are scheduled to be airlifted to affected areas, Xinhua reported. Another 3,000 police officers have been activated.
"It looks like they've mounted a pretty monumental effort to do the best that they can there," said Kate Janie, director of Mercy Corps, a humanitarian group channeling disaster aid to the region through a partner agency.
"I think the Chinese government will make very active, proactive, transparent steps in dealing with this."
Zhenyao said 60,000 tents and 50,000 quilts have been dispatched to the disaster zone.
Chengdu Shuangliu International Airport reopened Tuesday after authorities inspected its runways for damage following the quake, Xinhua reported. The resumption of air service gives the province additional links for funneling supplies into the badly battered region.
A 40-car freight train, carrying 13 tankers full of gasoline, derailed and caught fire Monday in Gansu province, officials said, according to state-run media, cutting the Baoji-Chengdu railway.
Monday's quake shook the ground in Beijing, 950 miles (1,528 km) away. Residents of the capital, which hosts this year's Olympic Games in August, said they felt a rolling sensation that lasted about a minute. It resulted in the evacuation of thousands of people from Beijing buildings.
A spokesman for the Beijing Olympic Committee said no Olympic venues were affected.
Previously: More than 7,600 people have been killed by Monday's powerful earthquake in just one affected region of central China, the Chinese government said.
State-run news agency Xinhua said the official toll had risen to 7,651 in Sichuan Province.
In addition, at least 48 people were killed in the northwest Gansu Province, Xinhua said.
Authorities had earlier said they believed about 10,000 people were injured in Beichuan County in the northeastern part of the province.
The Sichuan provincial disaster relief headquarters said 80 percent of the buildings collapsed in the Beichuan Qiang Autonomous County after the 7.8-magnitude quake, Xinhua reported.
Several hundred students were also feared to be buried in collapsed school buildings, the agency said.
China's Seismological Bureau said the earthquake had affected more than half the country's provinces and municipalities.
President George W. Bush released a statement saying the United States "stands ready to help in any way possible."
"I am particularly saddened by the number of students and children affected by this tragedy," Bush said.
In Sichuan's Shifang city, the quake buried hundreds of people in two collapsed chemical plants, and more than 80 tons of ammonia leaked out, Xinhua said.
The local government evacuated 6,000 civilians from the area after homes and factories were also destroyed.
The quake was "felt in most parts of China," Xinhua reported, with the confirmed casualties in the provinces and municipality of Sichuan, Gansu, Chongqing and Yunnan.
Xinhua said several schools collapsed, at least partially, in the quake.
At one, as many as 900 students were feared buried. At least 50 bodies have been pulled from the rubble at the high school in the Juyuan Township of Dujiangyan City in Wenchuan County.
"Some buried teenagers were struggling to break loose from underneath the ruins while others were crying out for help," Xinhua reported.
"Grieved parents watched as five cranes were excavating at the site and an ambulance was waiting.
"An unknown number of students were also reported buried after buildings collapsed at five other schools in the province's Deyang City."
One person was killed in Santai County, in the city of Mianyang, when a water tower fell, the news agency reported.
A provincial government spokesman said they feared more dead and injured in collapsed houses in Dujiangyan City, Xinhua reported.
The news agency also quoted a driver for the seismological bureau saying he saw "rows of houses collapsed" in Dujiangyan.
Chinese President Hu Jintao immediately ordered an all-out effort to help victims of the earthquakes, Xinhua reported. It said Premier Wen Jiabao would go there to direct the rescue work.
Bonnie Thie, the country director the Peace Corps, was on a university campus in Chengdu about 100 km from the epicenter, in the eastern part of China's Sichuan province, when the first quake hit.
"You could see the ground shaking," Thie told CNN.
The shaking "went on for what seemed like a very long time," she said.
"This is a very dangerous earthquake," said Bruce Presgrave, a geophysicist with the U.S. Geological Survey.
The quake had the potential to cause major damage because of its strength and proximity to major population centers, he said.
In addition, the earthquake was relatively shallow, Presgrave said, and those kinds of quakes tend to do more damage near the epicenter than deeper ones.
An earthquake with 7.5 magnitude in the northern Chinese city of Tangshan killed 255,000 people in 1976 -- the greatest death toll from an earthquake in the last four centuries and the second greatest in recorded history, according to the U.S. Geological Survey.
Tangshan is roughly 1,600 km from Chengdu, the nearest major city to the epicenter of Monday's quake.
After the first quake struck Monday, the ground shook as far away as Beijing, which is 1,500 km from the epicenter.
They felt "a very quiet rolling sensation" that lasted for about a minute, according to CNN correspondent John Vause.
"Our building began to sway," he said.
Thousands of people were evacuated from Beijing high-rises immediately after the earthquake.
At least six more earthquakes -- measuring between 4.0 and 6.0 magnitudes -- happened nearby over the three hours after the initial quake at at 2:28 p.m. local time (0728 GMT), the USGS reported.
A spokesman for the Beijing Olympic Committee said no Olympic venues were affected by the earthquake. The massive Three Gorges Dam -- roughly 600 km east of the epicenter -- was not damaged, a spokesman said.
The earthquake was also felt in Hong Kong, Shanghai, Taiwan, and as far away as Hanoi, Vietnam, and Bangkok, Thailand, according to the Hong Kong-based Mandarin-language channel Phoenix TV.