At least 22 people - most of them babies and toddlers - are now believed to have contracted polio in Syria, the World Health Organization has reported.
If confirmed, it would be the first outbreak of the disease there in 14 years. Syria's Health Ministry began an immunisation drive on Thursday.
Before Syria's civil war began in 2011, some 95% of children were vaccinated against the disease.
Now, Unicef estimates 500,000 children have not been immunised.
WHO said the suspected outbreak centres on the eastern province of Deir Ezzor.
"There is a cluster of 22 acute flaccid paralysis cases that is being investigated in that area," WHO spokesman Oliver Rosenbauer told Reuters news agency. "Everybody is treating this as an outbreak and is in outbreak response mode."
Two cases have already been confirmed by laboratory tests while the WHO expects final laboratory confirmation on the remaining 20 cases next week.
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.
The WHO is now working with the UN, Syria's Health Ministry and other agencies on a mass immunisation programme.
But it is expected to be a difficult undertaking, says the BBC's Imogen Foulkes in Geneva, given the widespread insecurity and estimates that over half of Syria's medical professionals have left the country.
More than four million Syrians have been displaced internally by the conflict and generally live in overcrowded, unsanitary conditions. The WHO has already reported increases in cases of measles, typhoid and hepatitis A.
Aid agencies are also developing emergency immunisation plans for Syrian refugee camps in Jordan, Lebanon, Turkey and Egypt.
"Obviously, we're extremely worried about the situation," Simon Ingram, a spokesman for Unicef's operations in the Middle East, told the BBC.
"People are flooding across borders in an uncontrolled manner and this increases the possibilities and means by which the virus can spread."
No known cure
The highly contagious disease is most often spread by consuming food or liquid contaminated with faeces.
Polio has been largely eradicated in developed countries but remains endemic in Nigeria, Pakistan and Afghanistan.
Worldwide, polio cases have fallen from an estimated 350,000 at the start of a WHO-led immunisation campaign in 1988 to just 223 reported cases last year.
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.