BEIJING: At least three dozen villages and towns in southwest China remained cut off from the outside world Thursday as tens of thousands of soldiers and emergency workers struggled against impassable roads and mountains of concrete and brick to reach the 40,000 people that officials say are still buried in the rubble or missing after a massive earthquake Monday. With the official death toll at more than 19,500 but expected to keep escalating, Prime Minister Wen Jiabao dispatched an additional 90 helicopters to the quake-battered Sichuan Province and sent out a national plea for heavy equipment and simple tools like hammers and shovels. Local officials in Sichuan also issued a radio appeal for food, water and heavy machinery, warning that a looming humanitarian crisis threatened thousands of quake survivors who have little access to food, clean water and shelter. On Thursday afternoon, officials from the largest Chinese power company warned that two dams were at risk of crumbling, threatening to drown more than 100 people still trapped in the ruins of a hydropower plant in the town of Huaneng. The dam, officials said on state-run television, could collapse "at any time." As a series of aftershocks rattled the area, officials warned of a potentially greater threat: an additional 391 dams were in "dangerous condition," posing an imminent risk to thousands of people downstream, according to the National Development and Reform Commission. A report in the Beijing Times said that a dam along the Jian River in Beichuan County was also in a fragile state. "There are major safety issues right now with the reservoirs, hydropower stations and lakes in the earthquake zone," Chen Lei, a government minister, said in the statement on the agency's Web site. "The area has numerous reservoirs and lots of damage, and the extent of the danger is unknown," Chen said. Overall, officials said that more than half of the region's 20 million people have been affected by the earthquake, which struck Monday afternoon with a magnitude of 7.9. Officials raised the number of injured to 65,000, with more than 1,600 of them in serious condition, Xinhua, the official news agency, reported. Relief experts said that time was quickly running out for those still buried in the rubble of collapsed buildings, including hundreds of students and teachers trapped in several schools throughout the region. For the first time since the earthquake struck, rescue workers cleared a major road leading into the hard-hit area around Shifang, enabling heavy equipment to gain access. The Associated Press reported that soldiers in Shifang could be seen bundling bodies in white sheets and burying them in a mass grave sprinkled with lime. Officials said that more than 130,000 emergency personnel, including soldiers and medics, were working in the quake zone. The government also said that it planned to send 1,300 rail cars with supplies to the region. The state media also reported on Thursday that 33 tourists from France, Britain and the United States had been airlifted by helicopter from the Wolong nature preserve, which is home to more than 100 pandas. Tourist officials said that an additional 2,500 tourists, including 682 foreign residents, would soon be evacuated. China asked Japan to send rescue workers and also gave the green light to a contingent of quake-relief experts from Taiwan and Hong Kong. Taiwan's largest private carrier, China Airlines, was given permission to ferry relief supplies on charter flights. "Time is life," Wen told survivors during a visit to Beichuan on Wednesday. "This is very important to let them know rescuers are trying to save them." Until now, the authorities have been reluctant to allow international aid workers into the quake zone, which is home to several military installations and China's nuclear-weapons design program. On Wednesday, officials declined an offer of help from the Australian government. Kate Janis, program director for Mercy Corps, an American organization that has personnel in Chengdu, the provincial capital, said she had been impressed by the relief efforts so far. "It definitely appears to be a no-holds-barred effort, all hands on deck," she said. A two-stage quake The fault line that caused this week's devastating earthquake in China probably buckled in two stages, and the hardness of the terrain contributed to the wide reach of the damage, Japanese scientists quoted by The Associated Press in Tokyo said Thursday. The quake in Sichuan Province rattled buildings as far away as Beijing, Shanghai and Thailand. Yuji Yagi, a seismologist at Tsukuba University, said that data show the Longmenshan Fault, which is 250 kilometers long, or 155 miles, tore in two sections, the first one ripping about 6.5 meters, or about 21 feet, followed by a second one that sheared about 3.5 meters. Despite the two-stage quake, which Yagi estimated lasted for about two minutes, it was the shallowness of the epicenter - only 10 kilometers deep - that contributed most to the destructive power, he said. The other distinguishing factor was the firmness of the terrain in central China, which allowed seismic waves to travel large distances without losing their power, a Tokyo University seismologist, Teruyuki Kato, said. Yagi calculated that the energy level of the China quake was 30 times that of the one that struck Kobe in January 1995.