SHANGHAI, June 30 (Reuters) - A new flu virus found in pigs in China has become more infectious to humans and needs to be watched closely in case it becomes a potential pandemic virus, a study said, although experts said there is no imminent threat.

A team of Chinese researchers looked at influenza viruses found in pigs from 2011 to 2018 and found a G4 strain of H1N1 that has all the essential hallmarks of a candidate pandemic virus, according to the paper, published by the U.S. journal, Proceedings of the National Academy of Sciences (PNAS).

Pig farm workers also showed elevated levels of the virus in their blood, the authors said, adding that close monitoring inhuman populations, especially the workers in the swine industry, should be urgently implemented.

The study highlights the risks of viruses crossing the species barrier into humans, especially in densely populated regions in China, where millions live in close proximity to farms, breeding facilities, slaughterhouses and wet markets.