Covid-19 has killed nearly 2,600 people in mainland China, where the virus emerged last year, and infected more than 77,000.
More than 1,200 cases have been confirmed in 30 other countries, with Japan, South Korea and Italy among those experiencing large outbreaks and increasing infection rates.
It has raised concerns among officials and scientists that efforts to tackle the coronavirus by enforcing quarantines are not working.
In the scientific journal Cell, Marion Koopmans, a professor of viroscience at Erasmus University and WHO advisor, said: “Whether it will be contained or not, this outbreak is rapidly becoming the first true pandemic challenge that fits the disease X category, listed to the WHO’s priority list of diseases for which we need to prepare in our current globalized society.”
Disease X is a placeholder name adopted by the WHO for any new pathogen which may cause disease and potentially an epidemic in the future but is not yet known to scientists.
Dr Koopmans also accused public health experts and authorities of “wasting precious time” for not being better prepared against diseases that hold the potential to become pandemic.
However the head of the WHO attempted to reassure the public that coronavirus could still be contained, while acknowledging it had the “potential” to become a pandemic.
“Using the word pandemic now does not fit the facts, but it may certainly cause fear,” said Tedros Ghebreyesus.
“We must focus on containment, while doing everything we can to prepare for a potential pandemic. There is no one-size-fits-all approach.”
Dr Ghebreyesus said the sudden increase in cases in Italy, Iran and South Korea were “deeply concerning” but added: “For the moment, we are not witnessing the uncontained global spread of this virus, and we are not witnessing large-scale severe disease or death.”
On Monday, health officials announced that a seventh person had died in Italy after contracting Covid-19. More than 220 people in the country have been infected with the virus since Friday, according to the latest data, the vast majority of them in the regions of Lombardy and Veneto.
Kuwait, Bahrain, Oman and Iraq have also recorded their first new coronavirus cases, all people who had been in Iran, which raised its toll from the disease to 12 dead and 61 infected.
Ian Mackay, an associate professor of virology at the University of Queensland, has suggested that the recent outbreaks outside China may be just the tip of the iceberg.
“Those countries are canaries in the coalmine that the virus is quite active — a sign that containment is reaching the end of its applicability,” Mr Mackay said. “There could be these sorts of spot fires burning everywhere with us not knowing.”
Another study, published last week by Imperial College London, said that about two-thirds of coronavirus cases exported from China have not been detected yet.
Additional reporting by agencies