One in five commercial milk samples tested in a United States-wide survey contained particles of the H5N1 virus, the US Food and Drug Administration says, suggesting the outbreak of bird flu is more widespread than previously thought.
The agency said there is no reason to believe the virus found in milk poses a risk to human health.
"This says this virus has largely saturated dairy cattle throughout the country," said Dr Michael Osterholm, an infectious disease expert at the University of Minnesota.
Many infectious disease experts and government officials have said they believe the pasteurisation process will inactivate the virus, also known as avian influenza.
However, additional testing is needed to confirm that there is no infectious virus in the milk, the agency said.
"To date, the retail milk studies have shown no results that would change our assessment that the commercial milk supply is safe," the FDA said in its latest update.
"I'm not worried about the milk itself. It does indicate that the virus is more widespread among dairies than we had previously thought," said Samuel Alcaine, associate professor, of food science at Cornell University.
"We had a little over 30 herds or farms that had been reported as having positive for avian influenza. We have just under 30,000 farms across the US. Thirty-three is a really small number. It makes it seem like there is definitely more spread out there."
Eight US states have confirmed cases of bird flu in 33 dairy herds, according to the US Department of Agriculture.
Only one person - a Texas farm worker - has been confirmed to have bird flu in the current outbreak.
The patient suffered conjunctivitis, an eye irritation that can cause redness and discomfort.
The FDA said the US Centers for Disease Control and Prevention has not recorded additional human cases beyond the first confirmed case.
FDA is further assessing any positive findings through egg inoculation tests, which it described as a gold standard for determining viable virus.
The World Health Organisation said on Friday that the current overall public health risk posed by H5N1 bird flu virus is low but for those with exposure to infected birds or animals the risk of infection is considered low to moderate.
"The virus has been detected in milk and its potential role in transmission is being investigated," the agency added.
