The WHO on Friday says the H5N1 bird flu virus strain has been detected in very high concentrations in raw milk from infected animals, though how long the virus can survive in milk is unknown.
Cows and goats joined the list last month, a surprising development for experts because they were not thought to be susceptible to this type of influenza.
US authorities earlier this month said a person working on a dairy farm in Texas was recovering from bird flu after being exposed to cattle.
It was only the second case of a human testing positive for bird flu in the United States, and it came after the virus sickened herds that were apparently exposed to wild birds.
The Texas health department says the cattle infections do not present a concern for the commercial milk supply, as dairies are required to destroy milk from sick cows.
From 2003 to April 1 this year, the WHO said it had recorded 463 deaths from 889 human cases across 23 countries, putting the case fatality rate at 52 percent.