The UN health agency on Tuesday confirmed an outbreak of polio in war-torn Syria, which had been free of the crippling disease since 1999, and said it feared it would spread.
Oliver Rosenbauer, spokesman for the World Health Organization's anti-polio division, told reporters that laboratory tests had confirmed the presence of the disease in 10 out of 22 suspected cases reported in children almost two weeks ago.
All 22 children were stricken with acute flaccid paralysis, which is the symptom of a number of different diseases, including polio.
"In 10 of those cases, they've isolated wild polio virus type one," Rosenbauer said.
"The other 12 are still being investigated," he added, saying test results were expected in coming days.
Thanks to a global drive against polio, the virus is now endemic in just three countries: Afghanistan, Pakistan and Nigeria.
That has raised speculation that foreign jihadists battling President Bashar al-Assad could have brought the virus to Syria.
WHO officials declined to comment on that speculation.
But US officials said Washington was "extremely concerned by the outbreak of polio, especially in view of the decline of medical services in the humanitarian crisis in Syria."
State Department spokeswoman Jen Psaki urged all sides "to allow access for polio and other needed vaccinations and other humanitarian assistance to those in need in Syria."
Rosenbauer stressed that even if polio is endemic only in three countries currently, the virus could have arrived from elsewhere.
For example, there is currently an outbreak in the Horn of Africa.
"Anybody can carry the virus, and that's the big danger as well, that it can go very, very widely," Rosenbauer told AFP.
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"This is the problem with this disease: unless you eradicate it from the remaining endemic areas you are going to run the risk of seeing polio re-emerge in polio-free ones," he said, noting that outbreaks occur "time and again" around the globe.
"Countries with complex emergencies like Syria or Somalia are particularly at risk because the health systems deteriorate and immunisation levels deteriorate," he added.
The Syrian cases were clustered in the northeastern Deir Al Zour province, and all affected children are under the age of two.
"There are no additional 'hot' cases that we know of. Of course disease surveillance is now ongoing across Syria and neighbouring countries as well, to look for other acute flaccid paralysis cases," said Rosenbauer.
"We consider a single case an outbreak because you have to assume that others are infected by the disease who are not showing symptoms... By the time you find one case, the horse is out of the barn," he added.
An analysis of the genetic code of the virus is underway to try to track its source, and the result should be known by next week, Rosenbauer said.
Last week aid agencies and Syrian health authorities stepped up efforts to vaccinate 2.4 million children against polio, as well as measles, mumps and rubella.
Before the conflict began in 2011, around 95 percent of all Syrian children were vaccinated against polio.
The United Nations says that 500,000 children there have not been vaccinated against polio in the past two years.
Rosenbauer said that all the children who have caught the virus in Deir Al Zour appeared to have never been vaccinated against polio, or had not received a full course of the vaccine.
Outside Syria, vaccination campaigns which had been planned in refugee camps will have to be broadened to head off the risk of a wider outbreak, he said.
"Ideally you want to target all children under the age of five across the region," said Rosenbauer.
An estimated 115,000 people have been killed in Syria and millions driven from their homes since a brutal crackdown on peaceful pro-democracy protests in March 2011 escalated into civil war.
Of the more than two million Syrians who have fled abroad, most are in neighbouring Lebanon, Jordan, Turkey and Iraq.