A polio outbreak in war-ravaged Syria has paralysed at least 17 children since March, the World Health Organization said Tuesday, 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 3 and May 23, 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 group 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 IS control.

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

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

Vaccine-derived polio

Unlike the last in Syria, which affected 36 children, the current outbreak does not involve the so-called "wild" version of the virus.

Instead, it originated from a specific type of polio vaccine, which contains small amounts of weakened but live .

Oral vaccine (OPV) replicates in the gut and can be passed to others through faecal-contaminated water—meaning it won't hurt the child who has been vaccinated, but could infect their neighbours in places where hygiene and immunisation levels are low.

WHO plans to bring in more of the vaccine to get immunisations levels high enough to ensure they can halt the , Jasarevic said, adding that the aim is to vaccinate more than 400,000 under five in Deir Ezzor.

"Our teams on the ground are currently looking into the logistics," WHO spokesman Oliver Rosenbauer told AFP.

"We still don't know how we will get the in... by truck or helicopter," he added.