BEIJING: Four days after a powerful earthquake devastated a mountainous region of southwestern China, the nation's massive rescue and relief effort continued Friday, even though the hope of finding new survivors was dimming. Remarkably, relief officials said that they had rescued a child buried alive in the ruins of a middle school late Thursday, about 80 hours after the quake struck, and had also detected the sounds of several other children who could be trapped there. A 23-year-old nurse was pulled from the rubble Friday, more than 90 hours after the quake, in Beichuan County, a severely damaged area north of Chengdu, the provincial capital of Sichuan. But relief officials know the time for miracles is running out. The death toll in the country's worst natural disaster in 30 years surpassed 22,000 on Friday evening, and the government estimated that the figure could climb to as many as 50,000 people. The government also said Friday that it was investigating why so many school buildings had collapsed after the earthquake, killing hundreds or thousands of children. Beijing promised to mete out harsh punishment if any wrongdoing was involved. "If quality problems do exist in the school buildings, we will deal with the persons responsible strictly with no toleration," Ha Jin, an official with the Ministry of Education, told the state-run press. The government said it had already deployed more than 130,000 military and relief workers in devastated parts of Sichuan Province, where thousands are still believed to be buried in the ruins and where perhaps a hundred thousand or more have been left homeless. And, for the first time on Friday, Beijing allowed international relief experts to help in a domestic operation. A 31-member team from Japan arrived early Friday, and another team was on the way. Experts from Taiwan, South Korea and Russia were expected to arrive soon. But rescue efforts have been hampered all week by bad weather, treacherous mountain terrain and thousands of aftershocks, some of them quite severe. On Friday, the government said that a large aftershock in Li County, west of the epicenter, had created additional landslides, burying cars, disrupting communication and making it even more difficult for rescuers to reach certain areas. Thousands of soldiers and rescue officials were marching on foot to reach some devastated areas. Earlier this week, the air force parachuted rescuers into some of the more remote locations, near the epicenter, in the city of Wenchuan. The government is also scrambling to repair roads and assess damage to hundreds of dams, which officials have warned could pose further hazards. "Scientists and the government still cannot estimate the losses to dams and other infrastructure," said Liu Weixin, a urban development specialist at the China Society of Urban Economy in Beijing. "The government said two dams around Chuanbei were destroyed, but they cannot estimate the loss exactly. And one of China's biggest dams, the Dujian Dam, is still unknown, and that's the biggest concern." Health officials are warning that piles of rotting bodies could pose health problems and spread disease and epidemics; funeral officials are being transported into the region to help cremate bodies. Although there had been remarkable rescues of children and even a pregnant woman earlier in the week, each day has also brought scenes of horror and grief, laying bare the enormous scope and scale of the region's tragedy. On Friday morning, President Hu Jintao arrived in Sichuan to assess the damage, reaching the city of Mianyang, one of the worst-hit areas. "Quake relief work has entered the most crucial phase," the president said, according to the state-run news media. "We must make every effort, race against time and overcome all difficulties." The president joined Prime Minister Wen Jiabao, who since Monday evening has led rescue efforts in the province, ordering supplies, consoling survivors and promising that the government would do everything in its power to help find survivors and care for the wounded. It was Wen who insisted early in the week that the government would spare no effort to find survivors. And in recent days, soldiers have marched along impassable roads, where landslides have occurred, and outdoor hospitals have been set up to tend to the wounded. The government said that more than 60,000 people have been hospitalized, and nearly 160,000 were injured by the earthquake. The huge rescue effort has been aided, however, by an enormous outpouring of international aid and blood and donation drives in cities all over China. The government says more than $200 million has been donated to relief efforts, including large donations from corporations around the world. The French retailer Carrefour, just weeks ago the target of nationalist anger in China, was one of them, according to Reuters. The agency quoted Chen Bo, the national communications manager for Carrefour China, saying that the firm had so far donated 3 million yuan, or $430,000, in cash and 220,000 yuan in goods to the relief effort. Chen played down the impact of the protests against Carrefour last month on the decision to donate, Reuters reported, noting that the company had also contributed to relief efforts after previous disasters, including the crippling snowstorms that hit southern and central China in January. The government has also undertaken a plan to find homes for orphans of the disastrous earthquake, repeating a similar plan that was instituted in 1976 after 240,000 people were killed by a massive earthquake in the city of Tangshan. The government says adoption applications have risen this week, and have included an offer from an orphanage in Tangshan to take in up to 200 children.