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 write in the journal Proceedings of the National Academy of Sciences that measures to control the virus in pigs, and the close monitoring of swine industry workers, should be swiftly implemented.