BEIJING (Agencies) - More than 420,000 houses collapsed and dozens injured in quake-hit Sichuan province in southwest China after two fresh aftershocks hit the area on Tuesday afternoon, heaping destruction and terror on a region struggling to recover from the country's worst earthquake in decades. The authorities ordered the evacuation of 80,000 people as troops scrambled to hold back a dangerously swelling lake formed by this month's devastating earthquake. As the death toll from the May 12 quake jumped to 67,000, state press said that emergency workers aimed to move the residents from their homes near the so-called "quake lake" by midnight (1600 GMT) Tuesday. Around 70,000 people were evacuated earlier from Beichuan County as concern rose that the lake - formed when the quake triggered landslides that blocked the Jian river - could overflow and surge downstream. Heightening the sense of fear still stalking China's southwestern Sichuan province, a strong aftershock measuring 5.4 on the Richter scale rattled the area and was felt in the provincial capital Chengdu. About 30 minutes later another aftershock measuring 5.7 shook neighbouring Shaanxi province, China's state news agency Xinhua reported. Xinhua later said 420,000 houses had collapsed in Sichuan's Qingchuan County as a result of the aftershocks and 63 people there had been injured, six critically. He Yongnian, former deputy director of the China Seismological Bureau, was quoted by Xinhua news agency as saying that aftershocks could "last for two or three months," complicating already difficult quake relief efforts. In Beijing, cabinet spokesman Guo Weimin said the confirmed death toll stood at 67,183, with a further 20,790 missing. One of the most urgent tasks now is trying to prevent the lake - one of 35 said to be at risk - spilling over and swamping an area that is home to some 1.3 million people. Troops armed with dynamite are trying to blast channels through the debris blocking the river to reduce the level of the lake, thought to be holding some 130 million cubic metres of water. Liu Ning, the water resources ministry's chief engineer who is supervising the operation, said people were being moved for their own safety. "It's better for them to complain about the trouble that the evacuation would bring than to shed tears after the possible danger," he said. The lake is inaccessible by road and the teams of soldiers, engineers and police had to hike through remote and mountainous terrain. Li Huzhang, an engineer in the paramilitary People's Armed Police, said at least 50,000 cubic metres of debris would have to be removed but rescuers were aiming for 100,000 cubic metres to minimise the risk. However, predictions of further rain threatened to make the operation more difficult and caused jitters among locals living in makeshift camps near their collapsed homes. Wang Sumu, 42, said the tents were "hot like ovens" in direct sunlight and "when it rains, they will be even more unbearable." Another concern is the possible spread of disease in the disaster zone. A health ministry spokesman said Tuesday that mass emergency vaccinations would be carried out, but reported no major outbreaks thus far. For the grieving survivors of the quake, there was some minor consolation after authorities announced that China's strict family planning policies would be eased to allow parents who lost a child to have another. New guidelines allow couples who lost their only child to have a second as long as they get official permission, the Chengdu Evening News said. Many parents are now demanding answers as to why so many schools collapsed so easily, amid allegations of lax government oversight that permitted shoddy construction work.