The World Health Organisation is working with the Cambodian authorities after two confirmed human cases of H5N1 bird flu were found among one family in the country.

Describing the situation as "worrying" due to the recent rise in cases in birds and mammals, Dr Sylvie Briand, the director of epidemic and pandemic preparedness and prevention, told reporters in a virtual briefing that the WHO was reviewing its global risk assessment in light of the recent developments.

The United Nations health agency last assessed the risk to humans from avian flu as low earlier this month.

Cambodian authorities on Thursday reported the death of an 11-year old girl due to H5N1, and began testing 12 of her contacts.

Her father, who had been showing symptoms, has also tested positive for the virus.

"The global H5N1 situation is worrying given the wide spread of the virus in birds around the world and the increasing reports of cases in mammals including humans," Briand said.

"WHO takes the risk from this virus seriously and urges heightened vigilance from all countries."

Briand said it was not yet clear whether there had been any human-to-human transmission, which was a key reason to focus on the cases in Cambodia, or if the two cases were due to the "same environmental conditions," likely close contact with infected birds or other animals.

A new strain of H5N1, clade 2.3.4.4b, emerged in 2020 and has been causing record numbers of deaths among wild birds and domestic poultry in recent months.

It has also infected mammals, raising global concerns.

However, unlike earlier outbreaks of H5N1, which has been around for more than two decades, this subtype is not causing significant illness in people.

So far, only about a half dozen cases have been reported to the WHO in people who had close contact with infected birds, and most of those have been mild.

Experts have suggested that the virus might need to change in order for human transmission to occur.

However, the WHO said it was stepping up preparedness efforts regardless, and noted that there were antivirals available, as well as 20 licensed pandemic vaccines if the situation changes although they would have to be updated to more closely match the circulating strain of H5N1 if needed.

That could take four to five months, said Richard Webby, director of the WHO Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds at St Jude Children's Hospital.

However, some stockpiled vaccines would be available in the meantime.

WHO-affiliated labs already hold two flu virus strains that are closely related to the circulating H5N1 virus, which manufacturers can use to develop new shots if needed.

A global meeting of flu experts this week suggested developing another strain that more closely matches H5N1 clade 2.3.4.4b, Webby told the briefing.