THE deadly bird flu virus could develop into a form that spreads easily between humans and cause a global outbreak, scientists have warned.

After studying 15 years of data on bird flu viruses in the wild, researchers said some strains of H5N1 avian flu were part way along the road to acquiring a handful of mutations needed to change into a form that could cause a devastating human pandemic.

"The remaining mutations could evolve in a single human host, making a virus evolving in nature a potentially serious threat," Derek Smith of Cambridge University, who led the research, told reporters.

Currently, bird flu, or H5N1 avian flu, can be transmitted from birds to birds, and birds to humans, but not from humans to humans.