BEIJING (AFP) - An aftershock with a magnitude of 5.3 on Thursday jolted an area of southwest China that was devastated by last month's earthquake, the US Geological Survey said. The aftershock rocked Qingchuan county in Sichuan province at 0441 GMT, the USGS reported on its website. The China Earthquake Administration said the aftershock measured 5.0 on the Richter scale. Two other aftershocks measuring 4.2 and 4.0 also rocked Qingchuan. No damages were immediately reported from Thursday's aftershock, which is one of over 10,000 to have hit Sichuan since the May 12 quake that registered 8.0 on the Richter scale. Meanwhile, Chinese Premier Wen Jiabao said that efforts to ease the dangers of the "quake lake" that threatens to burst its banks were at a critical juncture, as he flew there to oversee the relief work. "Now it's a critical moment for the Tangjiashan quake lake, and the most important thing is to ensure there is no casualty of the people," the official Xinhua news agency quoted Wen as saying. He arrived in the quake zone on Thursday morning and immediately boarded a helicopter to the lake at Tangjiashan, Xinhua reported. The lake was created in the May 12 earthquake when a landslide blocked the Jianjiang river, and the water masses have been building up steadily since then. The government is pinning its hopes on a channel dug by soldiers last week that is designed to drain at least enough water to contain the lake. An estimated 1.3 million people live in areas that could be inundated if the plan does not work, and plans are in place to evacuate them quickly if the lake bursts.