A polio outbreak in Syria has paralysed at least 17 children since March, the World Health Organization said Tuesday (June 20th), describing the situation as "very serious".

Fifteen more cases have thus been confirmed since WHO first announced, less than two weeks ago, that Syria had been hit by its first outbreak of the crippling disease since 2014.

"We are very much worried, because if there is one case of polio with a kid that is paralysed, it is already an outbreak," WHO spokesman Tarim Jasarevic told reporters in Geneva.

He pointed out that for every polio-caused paralysis, there are on average nearly 200 children who have the virus but no symptoms.

"The virus is circulating. It is very serious."

The new cases all surfaced between March 3rd and May 23rd, but were only just confirmed, since it can take up to two months to determine with certainty that a case of acute flaccid paralysis stems from polio, he said.

"It is expected that we will have more confirmations," Jasarevic warned.

All but one of the cases were registered in the Mayadeen district of the oil-rich Deir Ezzor province, most of which is controlled by the "Islamic State of Iraq and Syria" (ISIS) and where a siege of the capital has restricted access to basic goods and services for some of the population.

WHO did not specify whether the Deir Ezzor cases were in areas under ISIS control.

One case of the crippling and potentially fatal viral disease that mainly affects children under the age of five has also surfaced in ISIS stronghold al-Raqa.

But Jasarevic said it remained unclear if polio was actually circulating in al-Raqa, or if the paralysed child had caught the virus elsewhere.