A woman is critically ill in Hong Kong with the deadly H7N9 strain of bird flu, the first confirmed case in the city this winter, the Hong Kong government said.

The woman, 68, was admitted to hospital on Thursday after falling ill on Dec. 19, the Hong Kong government said in a statement issued on Saturday.

The case has been classified as "imported" after it emerged the woman had recently been in the Longgang district of Shenzhen in mainland China, where the first human case of the deadly H7N9 strain was reported in March 2013.

The woman ate chicken while in Shenzhen but was not exposed to live poultry, the government said.

The government is trying to track down friends of the woman who may have been exposed to the virus, the source of which had not been determined.

The first known human case of the H7N9 strain was reported in mainland China in March last year, according to the World Health Organization. The virus subsequently spread to Hong Kong last December where it killed three people.

Officials at Hong Kong's Health and Food and Environmental Health Departments were not available for comment.

Hong Kong hospitals have introduced measures to monitor and contain influenza infection, including cutting visitor hours, after the government raised the city's influenza pandemic response level to "serious", it said in a statement late on Saturday.