The new cases were registered in the oil-rich province Deir Ezzor, the majority of which is controlled by the Islamic State group and where a siege of the capital has restricted access to basic goods for some of the population.
An outbreak hit the province in 2013-2014, when 36 cases were reported, according to the Polio Global Irradication Initiative.
WHO spokeswoman Sona Bari told AFP on Thursday that the new cases are from a different strain of the virus.
The current form of the virus mutated from strains in the oral polio vaccine, the initiative said, as opposed to the "wild" polio which caused the last outbreak and led to the vaccination of millions of children in the Middle East.
"Circulating (vaccine-derived polioviruses) are extremely rare forms of poliovirus...that can emerge in under-immunised populations," it said in a statement.
"Although access to Deir-Ez-Zor is compromised due to insecurity, the Governorate has been partially reached by several vaccination campaigns against polio and other vaccine-preventable diseases since the beginning of 2016," the initiative said.
"Outbreak response plans are being finalized," it added.
Meanwhile medical journal The Lancet reported that health care workers, clinics and hospitals were attacked more than 400 times last year in Syria's civil war.
A total of 402 such attacks -- many of them fatal -- were registered between November 2015 and December 2016, it said.
