Hong Kong culled thousands of chickens Wednesday after the potentially deadly H7N9 bird flu virus was discovered in poultry imported from China, days after a woman was admitted to hospital with the disease.

Authorities found the virus in samples taken from 120 chickens imported from the nearby Chinese city of Huizhou and slaughtered nearly 19,000 birds, including 11,800 chickens.

"The rapid testing showed... that this batch of chickens carries the H7N9 virus," the city's health minister Ko Wing-man said Wednesday.

Televised images showed authorities beginning the cull Wednesday morning, with health officials in white hazmat suits dumping chickens into green plastic bins.

The bins were then pumped with carbon dioxide to kill the birds, a spokeswoman for the city's agriculture department told AFP.

The carcasses were sent to landfills for disposal after the operation was completed at around 5:30 pm.

Poultry imports from the mainland have been banned for three weeks.

A 68-year-old woman was admitted to hospital with the virus on December 25 after returning to Hong Kong from the neighbouring southern Chinese city of Shenzhen, although it has not been confirmed how she contracted the disease.

She remains in critical condition.

In response to the new case -- the city's first since early 2014 -- Hong Kong announced it was raising its response level in hospitals to "serious" from "alert", with extra precautions implemented from Sunday.

Ten people had previously been diagnosed with H7N9 in Hong Kong, including three who died. All had contracted the virus in mainland China, according to Hong Kong's Centre for Health Protection.

The H7N9 outbreak emerged on the mainland in February 2013.

Hong Kong last conducted a mass cull in January, slaughtering 20,000 chickens after the virus was found in poultry imported from the neighbouring southern Chinese province of Guangdong.

A four-month ban on live poultry imports from mainland China was imposed at the time to guard against the disease.

South Korea has culled millions of chickens this year in an attempt to stem the spread of bird flu. Japan this week ordered 37,000 chickens culled after the third bird flu outbreak there in less than a month.