Troops have arrived in Wenchuan county at the epicenter, which was largely cut off by the quake - but heavy rain is hampering rescue operations. Elsewhere in Sichuan province, frantic efforts are being made to reach thousands of people under the rubble.
The death toll is now more than 12,000, officials say, and looks set to rise.
In one city, Mianyang, near the epicenter, more than 18,000 people are said to be buried under the rubble and 3,629 have been confirmed dead, state news agency Xinhua reports.
In the nearby town of Mianzhu, at least 4,800 people are trapped under the rubble and massive landslides have buried roads to outlying villages, Xinhua says.
Premier Wen Jiabao was quick to reach the scene and urged rescuers to clear roads into the worst-hit areas as fast as possible.
"As long as there is even a little hope, we will redouble our efforts 100 times and will never relax our efforts," he told crying locals in the badly hit Dujiangyan city, south-east of the epicenter.