GUANGZHOU, Dec. 20-- Health authorities in south China's Guangdong Province are on high alert against H7N9 bird flu after four new human cases were confirmed this week.

From Sunday to Thursday, the province confirmed four new cases -- two in Yangjiang City, one in Dongguan and one in Shenzhen, raising the total cases in the province to six. The newly confirmed four are all in a critical condition.

"According to expert assessments, Guangdong faces an extremely high risk of sporadic human H7N9 infections this winter and next spring," said Zhang Yonghui, head of the provincial center for disease control and prevention.

The cases may occur at any time across the province and the Zhujiang River Delta region deserves special attention," Zhang added.

Guangdong confirmed its first human case of H7N9 in Huizhou City on Aug. 10 and the second in Dongguan on Nov. 5. The two patients -- a 51--year-old woman and a three-year-old boy -- have recovered after treatment in hospital.

On Dec. 11, three different environmental samples for a poultry market in the Longgang District of Shenzhen, tested positive for the H7N9 strain.

To reduce the chances of human infection and prevent live poultry markets from harboring the virus, strict management of live poultry markets should be put in place, said He Jianfeng, head of epidemic research at the disease control center.

Guangdong has entered a period of vulnerability to H7N9 and faces a severe prevention and control situation, according to Lin Shaochun, vice governor of Guangdong.

The province has urged local agricultural and forestry authorities strengthen daily management,tests and disinfection of live poultry markets and crackdown on the illegal wildlife trade.

Antiflu drugs such as Tamiflu should be used within 48 hours for those who show typical flu symptoms such as fever and have had contact with poultry, said Chen Yuansheng, director of the provincial health and family planning commission.

The Chinese mainland has reported more than 140 human cases of the deadly virus since it emerged in March, including 45 fatalities.