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 organization said on its Twitter account.
The incurable virus was confirmed this month in 13 children who became paralyzed. 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 paralyze a child within hours. This is Syria's first polio outbreak since 1999.
There is no cure for the highly infectious disease, it can only be prevented through immunization, usually three doses.
More than 20 million children are to be vaccinated in Syria and neighboring countries over the next six months, United Nations agencies say. Syria's immunization rates have plummeted from more than 90 percent before the conflict to around 68 percent.
The threat of the re-emergence of polio is not limited to countries near Syria, a paper released in the Lancet medical journal earlier this month said. Two German researchers warned that Syrian refugees fleeing the nearly three-year-old civil war there could introduce polio into Europe, especially in countries with relatively low vaccination coverage like Ukraine and Austria, if more is not done to contain the virus.