The national death toll from the quake has climbed past 13,000 and is likely to rise steeply after media said 19,000 people were buried in rubble in just one area of Sichuan.
A near overwrought Chinese Premier Wen Jiabao was shown on state television using a bullhorn to urge on rescuers.
"At present the number one thing is still saving people," Wen told local officials, according to Xinhua. "All collapsed buildings must be fully checked. If there is a glimmer of hope, then put everything into rescuing."
But the depth of destruction in the towns and worst-hit mountainous areas suggests that the influx of troops is likely to find many more bodies than survivors among the toppled buildings, which have become grim vigil sites for desperate families.
In Beichuan County, at least 1,000 students and teachers were buried under a seven-storey school building, and rows of apartment blocks in the town collapsed. Locals told Xinhua that up to 8,000 residents may have died.
"People escaped from the buildings but were only devoured by the landslides," one survivor, Lei Xiaoying, told Xinhua. "There was no way to escape."
That scene is repeated in many other places where troops are only now entering after battling rain and severed roads.
State media reported devastation in villages near the epicentre in Wenchuan, a remote county cut off by landslides about 100 km (60 miles) northwest of the Sichuan provincial capital, Chengdu.