China's commercial hub Shanghai is stepping up monitoring after a new strain of bird flu killed two people, state media said Monday, as Taiwan announced it would screen travellers from the mainland.

The government's National Health and Family Planning Commission said over the weekend that two men, aged 87 and 27, died in Shanghai in early March after being infected with H7N9 avian influenza.

A 35-year-old woman in the eastern province of Anhui, near Shanghai, was in critical condition after developing the sub-type which had not previously been transmitted to humans, it said.

Shanghai's health bureau has ordered hospitals to strengthen monitoring and supervision of respiratory illness cases, but authorities were unsure how the three became infected, the Shanghai Daily newspaper said on Monday.

The two victims in the city both had histories of chronic illness, said a statement from Shanghai's health bureau, without giving details.

None of their family or close associates had become infected with H7N9, although two sons of the elderly man both had respiratory illnesses around the same time and one of them died.

The woman from Anhui had contact with birds, while the younger man in Shanghai worked butchering and selling meat, the state-backed Health News said on its website on Monday, suggesting a route for transmission.

Some Internet users reacted with anger that the announcement came three weeks after the second man died, but authorities said time was needed to verify the new strain.

"Delaying the release of such information is gambling with people's lives," said a microblog post under the name Fenhu Kuaidi.

But Michael O'Leary, China representative for the World Health Organisation (WHO), said Beijing had reported the cases in a "timely" manner, having only definitely identified the virus on Friday.

"The question is whether the virus will mutate to become more infectious, human to human," he said. "There is no evidence of that at this time but viruses change all the time."

Taiwan, which saw 2.6 million Chinese visitors last year, said passengers from the mainland, Hong Kong and Macau must now pass a mandatory temperature check on arrival at its airports.

Taiwan residents who develop fevers with 10 days of travelling to China would also be required to undergo further health checks at hospitals, the island's disease control centre said late Sunday.

Hong Kong also said it would step up surveillance.

"We will heighten our vigilance and continue to maintain stringent port health measures in connection with this development," a spokesman for the city's Centre for Health Protection said.

One Chinese expert played down the threat from the new strain.

"So far, it is still an animal virus, not a human virus," the Shanghai Daily quoted Jiang Qingwu, head of the school of public health at Fudan University, as saying.

The more common H5N1 strain of avian influenza has killed more than 360 people globally from 2003 until March 12 this year, according to the WHO.

China reported two deaths from H5N1 in the southwestern province of Guizhou in February, the country's first cases so far this year.

China is considered one of the nations most at risk from bird flu because it has the world's biggest poultry population and many chickens in rural areas are kept close to humans.