A new strain of flu that has the potential to become a pandemic has been identified in China by scientists.
It emerged recently and is carried by pigs, but can infect humans, they say.
The researchers are concerned that it could mutate further so that it can spread easily from person to person, and trigger a global outbreak.
While it is not an immediate problem, they say, it has "all the hallmarks" of being highly adapted to infect humans and needs close monitoring.
As it's new, people could have little or no immunity to the virus.
The scientists wrote in a journal that measures to control the virus in pigs, and the close monitoring of swine industry workers, should be swiftly implemented.
A bad new strain of influenza is among the top disease threats that experts are watching for, even as the world attempts to bring to an end the current coronavirus pandemic.
The last pandemic flu the world encountered - the swine flu outbreak of 2009 that began in Mexico - was less deadly than initially feared, largely because many older people had some immunity to it, probably because of its similarity to other flu viruses that had circulated years before.
That virus, called A/H1N1pdm09, is now covered by the annual flu vaccine to make sure people are protected.
The new flu strain that has been identified in China is similar to 2009 swine flu, but with some new changes.
So far, it hasn't posed a big threat, but Prof Kin-Chow Chang and colleagues who have been studying it, said it is one to keep an eye on.
They found evidence of recent infection starting in people who worked in abattoirs and the swine industry in China.