Torrential downpours began on Thursday night and unleashed flash floods in Gansu province's Yuzhong, stranding some 4000 people, state broadcaster China Central Television (CCTV) reported.
In one mountainous area, precipitation had reached up to 195mm since early Thursday evening. Yuzhong county normally sees 300-400mm of rainfall for the entire year.
Yuzhong is situated among gullies and hills of wind-blown silt on one of the world's biggest loess plateaus. That makes it vulnerable to flash floods and landslides given the loose structure of the soil, which becomes unstable when saturated with water.
Ankle-high muddy water could be seen coursing down a hilly road flanked by uprooted trees in a video posted by CCTV.
"The top priority must be to make every possible effort to search for and rescue missing people, relocate and resettle people under threat, minimise casualties, and restore communications and transportation as quickly as possible," CCTV quoted Xi as saying.
Xi warned local governments not to succumb to "complacency and carelessness" in light of recent occurrences of extreme weather, CCTV said.
Record rainfall has lashed China's north and south in recent weeks in what meteorologists describe as extreme weather events linked to climate change.
Heavy rains and flooding have killed at least 60 people across northern China including Beijing since late July.
The National Development and Reform Commission said on Friday it has allocated 100 million yuan ($A21.34 million) to support rescue efforts in Gansu following the disaster.
China has announced at least six billion yuan of funding for disaster relief since April.
Earlier this week, Beijing allocated more than one billion yuan in disaster relief for Guangdong and the northern province of Hebei, as well as the capital, Beijing, and the northern region of Inner Mongolia, state news agency Xinhua said, including subsidies for damage to grain-growing areas.
"The rains will drive up prices for fresh fruits and vegetables," said Dan Wang, a China expert at Eurasia Group.
While some farmers might be able to exploit the situation to their benefit, agricultural losses would hit incomes as a whole, she added.
But the worst may be yet to come, with two to three typhoons expected to strike in August, emergency management authorities say.
The city of Foshan west of Guangzhou has been the epicentre of the province's chikungunya outbreak, while at least a dozen more have reported infections, which typically cause fever and severe joint pain, though deaths are rare.
The next few weeks are especially daunting for disease prevention and control, say provincial authorities, after the flood season, worsened by typhoons and heavy rain, boosted mosquito activity.
Spread by the bite of infected Aedes mosquitoes, global infections of the disease number at least 240,000 this year.
