Authorities say two people have died and a further 39 taken ill following the previously unseen outbreak.
But a new study has claimed the number of those affected is more likely to be approaching 2,000.
The claim came just hours before Chinese authorities revealed on Saturday they had discovered four more cases of pneumonia potentially linked to the outbreak.
The four individuals are now in hospital in Wuhan, the central city at the epicentre of the deadly infection.
For the new study, scientists took the number of cases reported outside China – two in Thailand and one in Japan – to infer how many were likely to have been infected in Wuhan, based on international flight data from the city’s airport.
“For Wuhan to have exported three cases to other countries would imply there would have to be many more cases than have been reported,” Professor Neil Ferguson, one of the report’s authors, told the BBC.
And he added: “I am substantially more concerned than I was a week ago.
“People should be considering the possibility of substantial human-to-human transmission more seriously than they have so far.”
Both the cause of the virus and the way it spreads remain unknown at present but authorities say it appears to be centred around a seafood market in Wuhan.
So far, neither health officials in China nor experts around the rest of the globe believe the disease – a coronavirus that affects the respiratory system – to be as lethal as SARS, but the investigation is evolving.
“This is the stage of the investigation where we need to proceed cautiously and be prepared for any eventuality,” Dr Nancy Messonnier, of the Centers for Disease Control and Prevention in the US, told the Reuters news agency.
Around much of the rest of the world – including in the UK – airports are continuing to screen flights with passengers arriving from Wuhan.
But, to add to potential complications, the outbreak comes ahead of the Lunar New Year holidays in China, which often sees what is called the world’s largest migration.
Some 440 million railway trips are expected to be made by people going home to family next week, along with 79 million flights abroad.