They say the G4 strain of the H1-N1 flu needs to be watched closely.

Professor Salim Abdool Karim says although the virus has a potential to become a pandemic, there's no need for concern for now.

"The concern is that there are an increasing number of individuals, particularly in the slaughterhouses, that have antibodies against this particular virus," Karim said.

"There's a concern it may have the potential to spread much more widely.

"At this stage, it does not look like its spreading much beyond the contained environment so there's no immediate concern."

Workers on pig farms showed elevated levels of the virus in their blood.

Authors of the report say while it's capable of infecting humans, there is no imminent risk of a new pandemic.

The World Health Organisation (WHO) meanwhile said it will "read carefully" the study on the new flu virus, saying the findings underscored the importance of influenza surveillance during the COVID-19 pandemic.

"We will read carefully the paper to understand what is new," the WHO's Christian Lindmeier told a Geneva briefing, saying it was important to collaborate on findings and keep tabs on animal populations.

"It also highlights we cannot let our guard down on influenza and need to be vigilant and continue surveillance even in the coronavirus pandemic," he added.

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