The WHO announced four new cases of the virus, three of which occurred in Saudi Arabia. The fourth was a man from Qatar who travelled to Germany for care.
The global agency said it is also in the process of reviewing its case definition to help health-care practitioners spot possible additional infections.
It advised countries to be on the lookout for possible cases, even in people who haven't travelled to Saudi Arabia and Qatar, the only two countries to date to have had citizens who have tested positive for the virus.
"Until more information is available, it is prudent to consider that the virus is likely more widely distributed than just the two countries which have identified cases," the WHO's statement says.
"Member states should consider testing of patients with unexplained pneumonias for the new coronavirus even in the absence of travel or other associations with the two affected countries."
The statement — and word on Twitter that the European Centre for Disease Control is planning to update its risk assessment of the coronavirus — suggests public health officials are worrying there are more instalments ahead in the story of this virus, a cousin of the coronavirus that cause SARS.
"Before we were wondering if these were really one-off transmissions which were just oddities in that they happened to occur around the same time," said Dr. Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.
"Now with these cases, you can't say it's just a very rare event."
The WHO said as of Friday there have been six confirmed cases of the infection, four from Saudi Arabia and two from Qatar. (Saudi officials had previously reported two of the cases to which the WHO statement refers.)
Two of the confirmed cases have died; both the fatal cases were Saudi citizens.
The Robert Koch Institute in Berlin informed the WHO of the latest Qatari case. A statement from the institute said the man recovered and was released this week.
All the confirmed cases have been men, WHO spokeswoman Fadela Chaib said in an email.
The new virus was first spotted in June, when a man from Saudi Arabia died after a serious respiratory infection. When the cause of his infection couldn't be identified, an infectious diseases specialist from the Jeddah hospital sent a specimen to Erasmus Medical Centre in Rotterdam, the Netherlands, which confirmed infection with a new coronavirus.
But word of the discovery of the new virus did not emerge until late September, around the time authorities in Britain were trying to diagnose a gravely ill man from Qatar who had travelled to London for treatment by air ambulance. The man, who is still in hospital in London, tested positive for the virus.
Up till now there has been no suggestion of person-to-person spread of the virus, prompting authorities to say the virus didn't pose a global threat. But this cluster in Saudi Arabia may change thinking on that.
Two men in a single household fell ill and tested positive for the virus. One of the two died.
Two other members of the same household were sick at the same time with similar symptoms; one of those men died as well. The survivor tested negative for the virus, but results are still pending on the testing of samples taken from the man who died, the WHO said Friday.
Chaib said if there was human-to-human spread in this case it looks like it petered out. She said work is underway to try to tease out whether the people were all infected from a single non-human source, or if one member of the household picked up the infection and passed it along.
"The timing of the cases in the Saudi cluster does raise that concern but when a cluster occurs in a setting such as a household where everyone has similar environmental exposures it can be very difficult to separate out exposure to the same environmental source versus spread from one person to another," she said.
"Investigations are on-going to try and answer this question, however if H2H (human-to-human spread) has occurred, it does not appear to be sustained."
Osterholm said too little information is known at present to be reassured that the negative test was a true negative. The reliability of the test could vary, depending on when the person was tested, what kind of test was used and the kind and calibre of the specimen being tested, he said.
"If the person had an illness similar to the other illnesses, then ... I believe that you'd have to consider that this test may have been a false negative," said Osterholm, adding testing the survivor's blood for antibodies would shed some light on the situation.
It's not clear what kind of testing has been done. In fact, very little information about the cases has been revealed.
The new statement does not mention the ages of the cases or when they became sick. It also does not say what symptoms the men suffered from, how they were treated or how sick the survivors were.
It does not reveal where the cases lived — in a city or in a rural setting, where they might be in closer proximity to animals that could be the source of the virus.
Last month teams of researchers from Columbia University in New York, the WHO and the U.S. Centers for Disease Control travelled to Saudi Arabia to investigate possible sources of the new virus. To date they have not publicly revealed whether they found any clues where the virus comes from or how people become infected with it.
The genetic blueprints of viruses recovered from the first two cases suggest this coronavirus comes from bats. But it is not known at this point whether the viruses jumped directly from bats to people — say through exposure to bat guano or urine — or from bats to other animals and then to humans.