GUANGZHOU, Jan. 29 -- Three more human cases of H7N9 bird flu have added the tally to 28 in south China's Guangdong Province since the start of the year, local health authorities said Thursday.

The three cases were reported on Wednesday in cities of Shenzhen, Foshan and Meizhou, the provincial health and family planning commission said in a statement.

A 21-year-old man from Shenzhen and a 59-year-old man from Meizhou were in stable condition, while a 42-year-old woman from Foshan was in critical condition.

Health experts have warned that Guangdong, which borders Hong Kong, has entered the high-incidence period for bird flu, which usually crops up in winter and spring seasons.

A series of human infection reports and the belief that most cases come from exposure to infected poultry have led to the culling of thousands of chickens in Guangdong and Hong Kong this year.

Several cities in Guangdong have also taken measures to restrict live poultry trade to curb the spread of the disease, raising concerns about the impact on the industry.

In addition to Guangdong, Shanghai, Fujian, Jiangxi and Zhejiang in east China have all reported human cases of H7N9 this winter.

H7N9 is a bird flu strain first reported to have infected humans in March 2013.