The Geneva-based agency says a cluster of more than 10 paralysis cases have been detected in Deir Al Zour province in eastern Syria, a contested area of the country.
The WHO's senior official for polio eradication says initial tests indicate polio is the cause, and efforts to address what could be a crisis situation are being geared up.
Dr. Bruce Aylward says the potential exists for a large scale outbreak that will take some time to bring under control.
That's because the collapse of health services during the civil war has meant there are young children in Syria who haven't been vaccinated.
Final confirmation may take a few more days, but Aylward says the WHO is assuming the worst.
"This quacks like a duck, looks like a duck — you've got to plan as though it's a duck," Aylward, the WHO's assistant director-general for polio, emergencies and country collaboration, said from Geneva late Saturday.
"We think that there's a high probability that this is real."
The unwelcome development is the latest setback for the global effort to eradicate polio, already 13 years behind schedule and billions of dollars over original cost estimates.
While polio remains endemic — meaning transmission has never been stopped — in only Pakistan, Afghanistan and Nigeria, several countries this year have seen spread of polio resume as a result of imported viruses. Among them is Somalia, where polio vaccination efforts were abandoned for four years because of conflict in the southern part of the country.
Conflict and disease outbreaks often go hand and hand. In Syria, the job of extinguishing this outbreak will likely be substantial, Aylward acknowledges.
"The potential is ... to have a large scale outbreak with a couple of hundred cases that takes some time to get back under control, very definitely," he says.
"If these cases are confirmed we're going to need six months of almost monthly campaigns to try and shut this down. "
Prior to the war, Syria had high vaccination coverage. But the Global Polio Eradication Initiative knows that there are children born since the unrest began in the spring of 2011 who will not have received even a single dose of polio vaccine.
"These are young kids, less than two years of age, and generally unvaccinated," Aylward says of the children in the cluster.
"To be honest, a lot of the data isn't good enough to know really how bad the underlying immunity situation might be right now."
Aylward, a Canadian, notes the WHO's emergency teams have been in Syria for the past three year and have a sense of the environment on the ground, the players, and the work that must be done to address the polio outbreak.
It won't be easy.
"Obviously this is the toughest environment anyone's going to have to work in right now," says Aylward.
He says a number of countries in the region — Egypt, Jordan, Lebanon, Iraq and Syria — were already planning major polio vaccination campaigns for November and December, before these possible cases were detected.