A new strain of bird flu that has killed 22 people in China is "one of the most lethal" of its kind and transmits more easily to humans than an earlier strain that has killed hundreds around the world since 2003, a top health official has said.

The H7N9 virus has infected 108 people in China since it was first detected in March, according to findings released on Wednesday by the Geneva-based World Health Organisation (WHO).

Although it is not clear exactly how people have been infected, WHO experts see no evidence so far of the most worrisome scenario; sustained transmission between people.

An international team of experts led by the WHO and the Chinese government conducted a five-day investigation in China, but said they were no closer to determining whether the virus could become transmissible between people.

Keiji Fukuda, WHO's assistant director-general for health security, said "the situation remains complex and difficult and evolving".

"When we look at influenza viruses, this is an unusually dangerous virus for humans," Fukuda said at a briefing, also naming the previous H5N1 strain that killed 30 of the 45 people infected in China between 2003 and 2013.

Although the H7N9 strain in the current outbreak has a lower fatality rate to date, he added: "This is definitely one of the most lethal influenza viruses that we've seen so far."

Fukuda stressed that the team was still at the beginning of its investigation and that "we may just be seeing the most serious infections" at this point.

'Harder to track'

The team of experts said what was mystifying about the latest virus was the absence of visible illness in poultry, "making it harder to track and control".

Fukuda also said that based on the evidence, "this virus is more easily transmissible from poultry to humans than [the] H5N1" virus, which has killed 371 people globally since 2003.

Ho Pak-leung, an associate professor in the Department of Microbiology at the University of Hong Kong, noted in the British Medical Journal that in the two months since it was first detected, the H7N9 flu has already resulted in almost twice as many confirmed infections in China as H5N1 caused there in a decade.

Besides the initial cases of H7N9 in and around Shanghai, others have been detected in Beijing and five provinces.

Samples from chickens, ducks and pigeons from poultry markets have tested positive for the H7N9 virus, but those from migratory birds have not, said Nancy Cox, director of the influenza division at the US Centres for Disease Control and Prevention.

"At least we can now understand the likely source of infection is poultry," Cox said.

There has been a "dramatic slowdown of cases" in the commercial capital of Shanghai, which has recorded most of the deaths, said Anne Kelso, the Melbourne-based director of the WHO Collaborating Centre for Reference and Research on Influenza.

After Shanghai closed down its live poultry markets in early April, "almost immediately there was a decline in detection of new cases," Kelso said.