Human cases of bird flu 'an enormous concern': WHO

Chickens in cages at a food market in Gansu province, Lanzhou, China. [AFP]

The World Health Organization voiced alarm Thursday at the growing spread of H5N1 bird flu to new species, including humans, who face an "extraordinarily high" mortality rate.

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