More than 10,000 people were moved to higher ground Thursday as water continued to rise in a brimming lake formed by landslides from China's May 12 earthquake and another strong aftershock rocked the quake-battered region.

There were no immediate reports of damage or injuries following the 5.3 aftershock in Sichuan province.

Meanwhile, Premier Wen Jiabao arrived by helicopter Thursday afternoon in the town of Mianyang downstream from Tangjiashan lake to oversee attempts to drain the water. It was his third trip to the quake zone.

"Now is a critical moment for the Tangjiashan quake lake, and the most important thing is to ensure there are no casualties," Wen was quoted as saying by the official Xinhua News Agency.

The lake was formed above the devastated town of Beichuan after rocks and soil blocked the route of the Tongkou river.

Xinhua reported that water was still more than four feet below a diversion channel carved to drain the lake and it was not clear whether other measures were being taken to control the steadily rising water level.

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"If it keeps raining heavily, Tangjiashan's water level will rise to the level of the flood-relief channel within two or three days, after which the water will naturally flow out through the channel," said a staffer reached by phone at the Sichuan provincial flood relief headquarters. The man declined to give his name because he was not authorized to speak to the media.

Xinhua reported that 10,441 people in a low-lying area 32 miles from the lake area were evacuated Thursday afternoon and a traffic ban was being enforced in areas downstream from the dam. According to Xinhua some 250,000 people have been evacuated from areas threatened by the lake.

Authorities had earlier begun evacuating people in the city of Mianyang, where tent camps set up to house quake victims were emptying out.

At a camp near Mianyang's Fule mountain, refugees said they had been told to leave for high ground by mid-afternoon.

"They told everybody to evacuate because it wouldn't be safe," said Yang Jiayun, sitting in her tent pitched near a hillside.

The U.S. Geological Survey said the aftershock struck at 12:41 p.m. just south of Qingchuan city at the relatively shallow depth of six miles.

The frequent aftershocks have exacerbated the Tangjiashan flood threat by sending waves crashing against the mud and rock blocking the river.

Steep hillsides that collapsed into river valleys during the massive earthquake blocked water and formed more than 30 new lakes throughout the area. Explosives and heavy machinery have been used in several cases to clear the rubble and divert water before it rises to threatening levels. Heavy rain in mountains upstream have also increased the risk.

Skies in Mianyang were clear at midday Thursday, although rain was expected by the weekend.

Elsewhere in the quake zone, parents whose children were crushed to death in their classrooms vowed to continue a grass-roots campaign for investigations into alleged corruption and shoddy construction.

On Wednesday, security forces blocked grieving parents and reporters from some collapsed schools in a sign that Beijing is becoming increasingly intolerant of their protests.

The quake has killed more than 69,000 people, but Lu Guangjin, spokesman for the State Council, China's Cabinet, said there was no tally of how many of the victims were schoolchildren. Another 17,991 people remain missing and about five million have been left homeless, forced to seek shelter in tent camps and hastily built prefabricated housing units.

The government has said about 7,000 classrooms were destroyed, while angry parents and even rescuers have pointed to steel reinforcing rods in broken concrete slabs that were thinner than a ball point pen.

Authorities promised to investigate the school collapses, but there has not been any word on the findings. Lu said officials were analyzing samples of the rubble but that the work would take time.

On Thursday, Qi Ji, vice minister of Housing and Urban-Rural Development, disputed the assertion that a disproportionate number of classrooms had collapsed, saying that "other public buildings and homes also collapsed." But he acknowledged that tougher building standards should be used in the reconstruction process.

Qi, who spoke at a regular news conference, said authorities are focusing on the goal of producing one million prefabricated temporary housing units within three months, a task that would require "round the clock" effort.