The United States reported the first outbreak of the deadly H7N9 bird flu on a poultry farm since 2017, as the country continues to grapple with another bird flu strain that has infected humans and caused egg prices to hit record highs.

The spread of avian influenza, commonly called bird flu, has ravaged flocks around the world, disrupting supply and fuelling higher food prices. Its spread to mammals, including dairy cows in the U.S., has raised concerns among governments about a risk of a new pandemic.

The strain that has caused most damage to poultry in recent years and the death of one person in the U.S. is H5N1.

But the H7N9 bird flu virus has proved to have a far higher death rate, killing nearly 40 per cent of the humans infected since it was first detected in 2013, the World Health Organization said.

The latest outbreak of H7N9 was detected on a farm of 47,654 commercial broiler breeder chickens in Noxubee, Miss., the Paris-based World Animal Health Organization said in a report on Monday, citing U.S. authorities.

"Highly pathogenic avian influenza (HPAI) H7N9 of North American wild bird lineage was detected in a commercial broiler breeder chicken flock in Mississippi. Depopulation of the affected flock is in progress," the report says.

The U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS), along with state animal health and wildlife officials, "are conducting a comprehensive epidemiological investigation and enhanced surveillance in response to the detection," it added.