The US Centers for Disease Control and Prevention said the threat to people from the virus remains low.
A man in Chile is infected with bird flu which has “worrying” mutations, US health officials have said.
The mutations may have taken place after the 53-year-old, who is hospitalized, fell ill.
But there is no evidence that the mutated virus has spread to other people, mixed with other flu viruses, or developed resistance to current drugs or vaccines.
The threat to people from the virus is low, said Vivien Dugan of the Centers for Disease Control and Prevention.
“Nevertheless, it is important to continue to carefully review each case of human infection,” she added.
“We need to stay alert to changes that would make these viruses more dangerous for people.”
Previous animal studies suggest that mutations could cause the bird flu the virus is more harmful or spreads more easily.
But there is no evidence that these mutations would make it easier for the virus to take root in a person’s upper lungs – a development that would raise concerns about the spread among people.
Similar mutations have been found in past bird flu infections.
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What is bird flu?
Bird flu, called type A H5N1, was first identified during an outbreak in Hong Kong in 1997 when visitors to live poultry markets caught it.
More than 450 people have died from bird flu in the past two decades, according to the World Health Organization.
The vast majority of infected people got it directly from birds.
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This comes as UK authorities announced protective measures against avian flu will be lifted next week.
The rules, which were introduced November 7 last yearhas made it a legal requirement to keep animals indoors and follow strict biosecurity measures to protect herds.
From April 18, bird farmers will once again be allowed to keep their flocks outdoors, and eggs laid by poultry kept outdoors can now be labeled as “free range”.
However, ornithologists have been urged to remain vigilant to avoid further outbreaks.
In the meantime, a 56-year-old woman in China died in recent days of a type of bird flu that is rare in humans.
She was the third person known to have been infected with the H3N8 subtype of bird flu.