(CNN) -- Additional polio cases have been confirmed in two new areas in Syria: near the capital of Damascus and in northern Aleppo, near Turkey, the World Health Organization said Tuesday.
In Syria's first outbreak of the disease since 1999, 10 cases of polio were initially confirmed among children last month. That number has now risen to 17, the WHO said.
In addition to 15 cases confirmed in Deir Ezzor province, "two additional cases have been confirmed, one each in rural Damascus and Aleppo, confirming widespread circulation of the virus," the WHO said in a statement.
It said the risk of further spread of the disease was considered high given the fighting, large-scale movement of refugees and number of children who have not been fully immunized in the country.
Last month, health officials launched a program to immunize 1.6 million Syrian children against polio, measles, mumps and rubella -- in government- and rebel-held areas. The response, which also includes neighboring countries, is expected to last at least six months, the WHO has said.
"A surveillance alert has been issued for the region to actively search for additional potential cases in addition to implementing the recommended supplementary immunization activities with oral polio vaccine," the WHO said.
About 22 million children under the age of 5 are being targeted in mass polio vaccination campaigns in the region, it said.
According to UNICEF, 500,000 children in Syria have not been vaccinated against polio.
The highly infectious viral disease, which spreads via fecal-oral transmission and contaminated food and water, primarily affects young children. Initial symptoms can include fever, fatigue, headache, vomiting, neck stiffness, limb pain and, in a small number of cases, paralysis and death.
It can be prevented through immunization, but there is no cure. The incidence of the disease has dropped by more than 99% since 1988. It remains endemic in three countries -- Pakistan, Nigeria and Afghanistan -- down from more than 125 countries in 1988.
Public health can be among the first casualties of war, as resources are diverted from ensuring clean water supplies and intact sewer lines.
According to the United Nations, more than 100,000 people have died in the Syrian conflict, which began in March 2011 when government forces cracked down on peaceful protesters.
CNN's Per Nyberg contributed to this report.