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 ISIS militants and a target of US-led airstrikes.
The WHO reported two polio cases in an area of Syria partly held by ISIS 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 UN 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 ISIS militants are trying to resist a US-backed assault.
Tensions escalated on Sunday as the US military brought down a jet near Raqqa for bombing near US-allied forces on the ground, and Iran launched missiles at ISIS targets in eastern Syria - the first time each state has carried out such actions in the multi-sided Syrian war.