Hong Kong began culling 15, 000 chickens on Wednesday and suspended imports of live poultry from mainland China for 21 days after the H7 bird flu strain was discovered in a batch of live chickens from the southern province of Guangdong.

Authorities also ordered the closure of the wholesale poultry market, where the virus was discovered, for 21 days for cleaning and disinfection.

The chickens infected with the H7 avian flu were imported from a farm in Huizhou city across the border from Hong Kong, said the Secretary for Food and Health Ko Wing-man.

The Agricultural, Fisheries and Conservation Department is culling all live poultry in the Cheung Sha Wan wholesale market, which is assigned to keep only imported poultry.

Hong Kong authorities usually cull birds by suffocating them in black bags filled with carbon dioxide.

"The department will conduct inspections as well as collect additional samples from all the 29 registered live chicken farms in Hong Kong to ensure that they are not affected by H7 influenza," Ko said.

Hong Kong confirmed its first case of deadly bird flu this winter at the weekend after a woman fell critically ill with the H7N9 strain of bird flu. It was suspected she caught the disease in the southern Chinese city of Shenzhen, where she recently visited and consumed chicken.

The city's authorities had culled 20,000 poultry in January this year when birds imported from the mainland were found to have the H7N9 strain.

China has had a problem with bird flu for several years, and new infections tend to crop up in the winter months.

The Xinhua state news agency said six people in the eastern province of Zhejiang had contracted the H7N9 bird flu strain this winter, one of whom had died.

Shanghai and the southeastern province of Fujian have also reported cases, the news agency added.

The H7N9 virus passes between birds, but there is not enough evidence to prove that it passes between humans, according to the World Health Organisation.

(Reporting by Venus Wu and Donny Kwok; Additional reporting by Ben Blanchard in BEIJING; Editing by Ryan Woo)