The warning, issued by the National Disaster Reduction Commission and the Ministry of Civil Affairs, urged local civil authorities to prepare for emergencies and minimise possible losses caused by the typhoon, the 29th to strike China this year.
The commission also asked provinces of Fujian, Guangdong and Hainan and Guangxi Zhuang Autonomous Region to strengthen vigilance, monitor any changes to Krosa's track and issue warnings in a timely manner.
Local authorities were also told to report any damage and relief progress to superior departments, as well as send relief materials and funds to ensure affected people are provided with the basic living necessities.
It will continue to strengthen as it moves northwest at a speed of eight km per hour, according to a forecast from the National Meteorological Centre.
Heavy rain and rainstorms have been forecast in the east of Hainan and central coastal Guangdong between Saturday and Monday, state-run news agency Xinhua reported.