Chinese Premier Wen Jiabao says efforts to drain a lake formed by last month's massive earthquake are reaching a critical moment as rising waters threaten to burst the naturally formed dam.

China's official Xinhua news agency says Mr. Wen flew over the so-called quake lake Thursday during this third visit to the earthquake zone in Sichuan province.

Mr. Wen says the most important thing now is to ensure there are no casualties.

More than one million people are estimated to be in areas that could be inundated if the natural dam breaks. Chinese authorities have already evacuated some 250,000 people from low-lying areas.

Chinese officials tell Xinhua that the quake-lake's waters, which are already close to overflowing, are likely to rise over the next two days. Rainfall is expected through Friday.

Officials hope a channel dug by soldiers last week will help drain the lake of its water and keep it from bursting.

The lake at Tangjiashan has become a pressing issue in the aftermath of China's 7.9 magnitude earthquake, which killed more than 69,000 people and left millions homeless.

Earthquake-battered Sichuan was rocked by another strong aftershock Thursday as Mr. Wen visited the region.

The U.S. Geological Survey says the aftershock registered 5.3 magnitude, and with an epicenter 65 kilometers west of the town of Guangyuan.

Meanwhile, Chinese authorities continued to block access to schools destroyed by last month's earthquake in an apparent effort to quell demonstrations by angry, grieving parents.

Police and soldiers have been deployed outside several schools in Sichuan, where parents are demanding official accountability for what they say is the shoddy construction of the facilities.

Speaking with reporters in Beijing Thursday, a top housing official denied that schools were poorly constructed and collapsed easier than other buildings.

Officials do say, however, that the quake-proof standards of construction for schools, hospitals and other buildings need to be improved.

Thousands of schools collapsed during the May 12 earthquake and by some estimates more than 6,000 students died.

Chinese officials have not said how many children died in the earthquake.

Some information for this report was provided by AFP, AP and Reuters.