In one city, Mianyang, as many as 18,645 people were buried in debris of factories, schools and hospitals, according to the official Xinhua news agency.
Rescue workers dug through demolished areas Tuesday and extricated 58 people from the debris so far, China Seismological Bureau spokesman Zhang Hongwei told Xinhua.
Storms and severed roads were hampering efforts to reach the mountainous region in central China rocked by Monday's 7.9 magnitude quake.
More than 12,000 people died in the southwestern Sichuan province and more than 26,000 were injured, the province's vice governor, Li Chengyun, told reporters, according to Reuters.
A 40-car freight train with 13 gasoline tankers derailed in the quake and was still burning Tuesday with no word on casualties, reported the Associated Press.
Aftershocks continued to rattle the region for a second day, sending people scrambling into the streets in the city of Chengdu.
Premier Wen Jiabao, visiting Sichuan, ordered troops to clear roads to the epicenter of the quake in Wenchuan.
"Please speed up the shipping of food. The kids have nothing to eat now," he said, Reuters reported.