A top scientist at the World Health Organization gave a dire warning about the dangers presented by the spread of bird flu to new species, noting that humans face an "extraordinarily high" mortality rate if infected, according to a report.

"This remains, I think, an enormous concern," the UN health agency's chief scientist Jeremy Farrar told reporters Thursday in Geneva, ScienceAlert reported.

WHO reported last week that of the 888 human cases of bird flu, H5N1, globally between January 2023 and March 2024, 52% of them were fatal.

The strain has become "a global zoonotic animal pandemic," Farrar said, ScienceAlert reported.

The bird flu outbreak began in 2020 and has killed tens of millions of birds. It has since evolved to infect cows and other mammals.

There is no evidence that it has been transmitted yet between humans.

"The great concern of course is that in ... infecting ducks and chickens and then increasingly mammals, that virus now evolves and develops the ability to infect humans and then critically the ability to go from human to human," he said.

Earlier this month, a person in Texas was diagnosed with an "extremely rare" case of bird flu after coming into contact with infected dairy cattle.

When "you come into the mammalian population, then you're getting closer to humans," Farrar said, adding that "this virus is just looking for new, novel hosts".

He recommended bolstering monitoring for bird flu, saying it is "very important understanding how many human infections are happening ... because that's where adaptation (of the virus) will happen."

"It's a tragic thing to say, but if I get infected with H5N1 and I die, that's the end of it. If I go around the community and I spread it to somebody else then you start the cycle," Farrar said.