Syria had launched a vaccination campaign around the country days after the Geneva-based WHO said it had received reports of children showing symptoms of polio in Syria's Deir el-Zour province, but the campaign faces difficulty with lack of access in many parts of the war-torn country.
Nearly all Syrian children were vaccinated against the disease before the civil war began more than 2 years ago. Polio was last reported in Syria in 1999.
The Syrian conflict, which began as a largely peaceful uprising against President Bashar Assad in March 2011, has triggered a humanitarian crisis on a massive scale, killing more than 100,000 people, driving nearly 7 million more from their homes and devastating cities and towns.
U.N. officials have warned of the spread of disease in Syria because of lack of access to basic hygiene and vaccinations.