The World Health Organization (WHO) has confirmed 10 cases of polio in war-torn Syria - the first outbreak in the country in 14 years.
The UN body 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 immunised.
Polio has been largely eradicated in developed countries but remains endemic in Nigeria, Pakistan and Afghanistan.
The BBC's Imogen Foulkes in the Swiss city of Geneva says there has been speculation that foreign groups fighting in Syria may have imported it.
WHO spokesman Oliver Rosenbauer said the source of the virus had to be one of the endemic areas.
"That's from where it would have always spread, and that's why it is so important to eradicate in those areas, because otherwise you are going to keep seeing polio in polio-free areas," he told the BBC.
The WHO said the suspected outbreak centres on the eastern province of Deir al-Zour.
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 Mr Rosenbauer as saying in Geneva.
"Immunisations 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 al-Zour province alone, which has been caught in fierce battles between Syrian government forces and opposition fighters.
The city of Deir al-Zour remains partially controlled by forces loyal to President Bashar al-Assad, while the countryside is in the hands of the opposition.
More than four 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.
"Anytime where you have areas with complex emergencies where health systems deteriorate, where immunisation levels deteriorate, children are much more vulnerable to diseases such as polio," Mr Rosenbauer told the BBC.
Since the first suspected polio case was reported 10 days ago, Syria's Health Ministry has begun an immunisation drive and aid agencies have begun developing emergency immunisation plans at Syrian refugee camps.
Mr 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 at the press conference.
There is no known cure for polio, though a series of vaccinations can confer immunity.
Young children are particularly susceptible to paralytic polio, the most serious form of the disease.