The WHO says a further 12 cases are still being investigated. Most of the 22 people who have been tested are babies and toddlers.
Before Syria’s civil war began in 2011, some 95% of children were vaccinated against the disease.
The UN now estimates 500,000 children have not been immunized.
The WHO said the suspected outbreak centres on the eastern province of Deir Ezzor.
The highly contagious disease is most often spread by consuming food or liquid contaminated with faeces.
“Of course this is a communicable disease, with population movements it can travel to other areas. So the risk is high for [its] spread across the region,” the Reuters news agency quotes WHO spokesman Oliver Rosenbauer as saying in Geneva.
“Immunizations have started in that area,” he said.
There are more than 100,000 children, all under age five, now at risk of polio in Deir Ezzor province alone, which has been caught in fierce battles between Syrian government forces and opposition fighters.
The city of Deir Ezzor remains partially controlled by forces loyal to President Bashar al-Assad, while the countryside is in the hands of the opposition.
More than 4 million Syrians have been displaced internally by the conflict and generally live in overcrowded, unsanitary conditions.
A further two million have fled the country, many of them living in refugee camps in Jordan, Lebanon, Turkey and Egypt.
The WHO has already reported increases in cases of measles, typhoid and hepatitis A.
Since the first suspected polio case was reported 10 days ago, Syria’s Health Ministry has begun an immunization drive and aid agencies have begun developing emergency immunization plans at Syrian refugee camps.
Oliver Rosenbauer said most victims were under two years old and were thought never to have been vaccinated against polio.
“The next step will be to look genetically at these isolated viruses and where they came from. That should give some clarity on the origin,” he said.
Polio has been largely eradicated in developed countries but remains endemic in Nigeria, Pakistan and Afghanistan.
There is no known cure, though a series of vaccinations can confer immunity.
Young children are particularly susceptible to paralytic polio, the most serious form of the disease.
What is polio?
Source: World Health Organization
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