A health worker administers polio vaccine to a child as part of a UNICEF-supported vaccination campaign in Damascus on Oct. 29. Omar Sanadiki/AP

The World Health Organization and UNICEF have launched a massive polio vaccination campaign aimed at immunizing 23 million children in the Middle East after 17 cases were discovered in Syria, the agencies announced Monday.

It will be the largest-ever immunization response in the region, WHO and the U.N. children’s fund said in a joint statement.

WHO has said the vaccinations would take place in Egypt, Iraq, Jordan, Lebanon, the West Bank and Gaza Strip, and Turkey.

Syrian refugees fleeing their country’s conflict have taken shelter in surrounding states, particularly Lebanon, Jordan, Turkey and Iraq.

“As if children in Syria had not suffered enough, they now have to contend with yet another threat to their health and well-being,” said Maria Calivis, UNICEF regional director for the Middle East and North Africa.

On Monday, UNICEF began the second part of its $30 million campaign, which started in November. In order to stop the outbreak and prevent further spread of the disease, organizers aim to vaccinate all children under the age of five.

The campaign is a response to the discovery of the first case of polio in Syria since 1999. The statement said at least 17 children had been paralyzed by polio — 15 of them in the eastern province of Deir Ezzor, large swathes of which are under rebel control.

The other two cases of the virus-borne disease were recorded in Aleppo in the north, and in Douma, near the capital Damascus.

“Inside Syria, the campaign aims to reach 2.2 million children, including those who live in contested areas and those who were missed in an earlier campaign,” the statement read.

The organizations warned that many children in Syria remain inaccessible because of the ongoing war. Vaccination will be offered at fixed sites at populous locations or by going from house to house.

“All Syrian children should be protected from disease,” said Ala Alwan, WHO’s regional director. “We appeal to all parties of the conflict in Syria to cooperate and facilitate pauses in hostilities over the coming six months to allow vaccination campaigns to reach all children.”

Health groups consider there is a high risk of polio spreading in the region.

The strain detected in Syria is related to a strain that originated in Pakistan and has also been detected in sewage samples in Egypt, Israel and the West Bank and Gaza Strip.

Al Jazeera and wire services