More than four million Syrians have been displaced internally by the conflict. BBC NEWS

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.

'Regional risk'

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.

"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 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 four million Syrians have been displaced internally by the conflict and generally live in overcrowded, unsanitary conditions.

A further two millions 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 case was reported 10 days ago, the 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, Reuters reports.

"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.