The World Health Organization said on Tuesday that additional polio cases had been confirmed in two new areas of Syria, including near Damascus and in the northern city of Aleppo near Turkey.
"In addition to 15 polio cases in Deir al-Zor province, Syria, two additional cases have been confirmed, one each in rural Damascus and Aleppo," the organisation said on its Twitter account.
The incurable virus was confirmed this month in 13 children who became paralysed. The WHO says polio is expected to spread after a drop in vaccination rates due to the war.
WHO spokeswoman Sona Bari said the virus had reached Aleppo, once Syria's most populous city where forces loyal to President Bashar al-Assad are fighting rebels trying to end four decades of family rule.
Polio is endemic in Pakistan, Afghanistan and Nigeria despite a 25-year-old campaign to eradicate the disease, which can paralyse a child within hours. This is Syria's first polio outbreak since 1999.
More than 20 million children are to be vaccinated in Syria and neighbouring countries over the next six months, United Nations agencies say. Syria's immunisation rates have plummeted from more than 90 percent before the conflict to around 68 percent.