Earlier this month, the agency reported two polio cases in the Mayadin area of Deir al-Zour province, much of which is controlled by so-called Islamic State.
Fourteen new cases have now been confirmed in the same area, while another was recorded in Raqqa province.
It is the first re-emergence of polio in the war-torn country since 2014.
The highly infectious disease, caused by a virus, mainly affects children under five years of age.
One in 200 infections leads to irreversible paralysis. Among those paralysed, 5-10% die when their breathing muscles become immobilised.
WHO spokesman Tarik Jasarevic told reporters that the 17 children with acute flaccid paralysis first showed symptoms between 3 March and 23 May.
But lab tests had only recently confirmed that the children had "vaccine-derived" polio, rather than the more virulent "wild" version of the virus, he said.
Oral polio vaccine (OPV) contains small amounts of weakened but live virus, which replicates in the intestine for a limited period and can be passed to others living in under-immunised areas through faecal-contaminated water or food.
Mr Jasarevic said the outbreak meant there was significant under-immunisation in the Mayadin area, and that in response the WHO planned to vaccinate 90,000 children under the age of five there and 320,000 others elsewhere in Deir al-Zour.
"We are very worried, because obviously if there is already one case of polio of a kid that is paralysed it's already an outbreak," he warned.
"We know for example that for one kid that is paralysed there are almost 200 asymptomatic so it means that virus circulating, so it is very serious."
Mr Jasarevic said the WHO was also carrying out a health assessment to ascertain whether the virus was circulating in Raqqa, where US-backed fighters are attempting to drive IS militants out of the provincial capital, or if the polio sufferer had caught the virus elsewhere and travelled there.
More than 300,000 people have lost their lives in six years of conflict in Syria, which began with protests against President Bashar al-Assad before escalating into a full-scale civil war. Eleven million other people have been displaced by the fighting.
Seventeen children have been paralysed by polio following an outbreak of the disease in Syria that the World Health Organization says is "very serious".