London

China's state-run news agency today said 420,000 houses had collapsed in aftershocks from the Sichuan earthquake.

The two tremors hit in Qingchuan county - one of the worst hit regions of Sichuan - and neighbouring Ningqiang county, in Shaanxi province, this afternoon. It is estimated that a total of five million people have been made homeless since the quake struck.

China's state-run news agency today said 420,000 houses had collapsed in aftershocks from the Sichuan earthquake.

The two tremors hit in Qingchuan county - one of the worst hit regions of Sichuan - and neighbouring Ningqiang county, in Shaanxi province, this afternoon.

It is estimated that a total of five million people have been made homeless since the quake struck.

Seismologists warned the aftershocks, although gradually weakening, could continue for months. One of today's aftershocks measured 5.7 magnitude, according to the US Geological Survey.

Another major threat comes from 34 "quake lakes" formed by landslides that dammed rivers during the main 7.9 magnitude quake a fortnight ago.

The biggest of these, the Tangjiashan lake, now contains 130m cubic metres of water and is rising by more than one metre every day, officials said.

The aftershocks came as soldiers used mechanical diggers to carve a 200-metre channel along the lake, two miles upstream from the centre of the devastated Beichuan county, in an attempt to release the build-up of water.

Other troops carried explosive charges through mountains to reach the lake and were preparing to clear the ground around its edge.

"We are prepared to get rid of the trees by chopping and explosion. After that, the second batch of equipment will be moved in," Liu Ning, the chief engineer at the water resources ministry, told state-run television.

The lake threatens more than one million survivors, and authorities are evacuating people who would be in the direct path of any flooding.

Emergency workers hope to move around 160,000 people by the end of today, the state-run Xinhua news agency said. If it appeared the entire dam would burst, 1.3 million would be moved.

Heavy rain is forecast, with almost three-quarters of the 60cm of rain that falls in Sichuan in an average year coming in the summer.

Officials in Beijing today said the total confirmed death toll was now 67,183, with 20,790 people sill missing.

Another potential hazard is disease. Health officials said higher than normal levels of stomach complaints and fever had been reported among quake survivors.

Millions of survivors are living in tents, but that as yet there have been no major disease outbreaks.

"With the destruction caused by the quake, the living and sanitary conditions have worsened for the local population," Qi Xiaoqiu, the head of disease prevention at the health ministry, said.

"Their physical conditions are weakened … [they are] more vulnerable to disease."

Diseases such as tuberculosis, hepatitis and diarrhoea remained a threat, he said.

Chinese leaders said that although they were focusing on resettlement and reconstruction, the search for survivors should not stop.

Many of those killed by the quake were children, and officials yesterday reassured bereaved parents they would be able to have another child despite China's strict one child policy.

Parents in Dujiangyan and Pengzhou, two badly-hit areas, will receive certificates allowing them to have another child if their first was killed, severely injured or disabled.

Officials also said couples who adopted earthquake orphans would still be allowed to have a biological child.

It is estimated that 4,000 children have been left without parents, some of whom have no other family.