The World Health Organization (WHO) logo is pictured at the entrance of its headquarters in Geneva, January 25, 2015. REUTERS/Pierre Albouy

GENEVA (Reuters) - Fifteen new cases of polio have been confirmed in Syria, including a child who may have caught the disease in Raqqa, the World Health Organization said on Tuesday.

Aid workers are unable to vaccinate the population in and around Raqqa, a city held by Islamic State militants and a target of U.S.-led airstrikes.

The WHO reported two polio cases in an area of Syria partly held by Islamic State earlier this month, the first re-emergence of the virus in Syria since 2014 and a blow for hopes of eradicating the disease globally.

WHO spokesman Tarik Jasarevic told a U.N. briefing on Tuesday that 14 more cases had been found in the same area, the Mayadin district of Deir al-Zor province, and another had come from Raqqa, a city where Islamic State fighters are trying to resist a U.S.-backed assault.

Tensions escalated on Sunday as the U.S. military brought down a jet near Raqqa for bombing near U.S.-allied forces on the ground, and Iran launched missiles at Islamic State targets in eastern Syria - the first time each state has carried out such actions in the multi-sided Syrian war.

“We are very worried, because obviously if there is already one case of polio of a kid that is paralyzed it’s already an outbreak. We know for example that for one kid that is paralyzed there are almost 200 asymptomatic so it means that virus circulating, so it is very serious,” Jasarevic said.

He said the WHO was doing a health assessment to ascertain whether the virus was circulating in Raqqa or if the polio sufferer had simply traveled there and caught the virus elsewhere.

The 17 children who have been paralyzed so far first reported symptoms between March 3 and May 23, and confirming the presence of the virus takes 6-8 weeks, meaning the outbreak could grow before its extent is known.

WHO plans to vaccinate 320,000 children under the age of five in Deir al-Zor and 90,000 in Mayadin, Jasarevic said.