Torrential rains slowed rescue efforts after a magnitude-6.5 earthquake killed at least 398 people in southwest China, as Premier Li Keqiang flew to the region to oversee the response to the disaster.

Three people remained missing in the region around the city of Zhaotong in the southwestern province of Yunnan, the official Xinhua News Agency reported. The quake, which hit yesterday afternoon, injured 1,801 and destroyed at least 12,000 homes, it said.

Efforts to help those injured and left without homes will be slowed because many roads were blocked and rain was forecast for the next four days. More than 2,500 troops have been sent to the area, and damage to bridges and roads meant rescuers had to reach affected areas by foot, according to Xinhua.

Chinese President Xi Jinping called for all-out efforts to reach those who may be trapped and to treat those injured. Li, who headed to the region, said the quake was the worst to jolt the region in 100 years, according to Xinhua

The quake, which was felt strongly in most of Yunnan, will cause “relatively serious destruction,” the China Earthquake Administration said in a statement. The tremor is the biggest in the province since a quake hit Yao’an county in January 2000, Xinhua reported, citing an unidentified person at the provincial earthquake bureau.

(Updated at 6 a.m. EST, Aug. 4)