Milk sold in US stores is "safe" from the bird flu because pasteurization effectively kills the disease, American health authorities said Friday, following spread of the infection among herds of cows.

An outbreak of Highly Pathogenic Avian Influenza (HPAI) has made its way through dairy cattle herds across the country and infected one human, who had mild symptoms.

On Thursday the FDA reported that initial results from a nationally representative commercial milk sampling showed about one in five retail milk samples tested positive for viral fragments, with a higher proportion in areas where herds are infected.

The FDA had announced earlier this week that thanks to the pasteurization process the virus was unlikely to present a health risk via milk. But it said additional tests were needed.

Preliminary results indicated "pasteurization is effective in inactivating HPAI," the FDA said in its press release Friday.

Bird flu was previously detected in raw milk, which health authorities have long advised against consuming.

Though the H5N1 strain of HPAI has killed millions of poultry during the current wave, affected cows have not fallen severely sick.

According to the FDA's Friday release, information from US health officials continues "to show no uptick of human cases of flu and no cases of H5N1, specifically, beyond the one known case."

There is no evidence of human-to-human bird flu transmission at this time, but scientists fear a mutation in the virus could allow it to pass among people.