More than 20 million children in Syria, Iraq and other countries in the region are to get polio vaccinations over the next five days.
The mass vaccination campaign is the first to cover Iraq, with the country last week confirming its first case in 14 years of poliomyelitis.
The contagious, crippling disease mainly affects children under 5 years. It cannot be cured but it can be prevented.
The move is part of an international response to a major polio outbreak in war-torn Syria confirmed in October - the first to hit the country since 1999.
The UN children's agency UNICEF said on Sunday 27 children in Syria had been paralysed by polio, most of them in the eastern province of Deir al-Zour.
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Syria's previously effective vaccination program has broken down under the strain of the ongoing civil war, which has seen most of the north and east fall under the control of fragmented rebel groups.
In addition to divided control on the ground and the destruction of medical infrastructure, more than 9 million of Syria's 22.4 million population have been uprooted during the three years of conflict.
Between 500,000 and 700,000 children have missed vaccinations as a result, UNICEF said.
The agency said it has already delivered 18 million doses of polio vaccine to Syria since the outbreak was confirmed, with five rounds of vaccinations carried out so far.
Chris Maher, the World Health Organisation's manager for polio eradication, said midway into the agencies' response, they were already "reaching the vast majority of children across the Middle East."
"In the second phase of the outbreak response we must work with local partners to reach the hardest-to-reach - those pockets of children who continue to miss out, especially in Syria's besieged and conflict areas and in remote areas of Iraq. We won't stop until we reach them," Maher said.
WHO recently declared India polio-free, leaving the disease endemic only in Afghanistan, Nigeria and Pakistan.
As part of the regional response to the Syrian and Iraqi outbreaks, vaccination campaigns are also under way in Egypt, Israel, Lebanon, the Palestinian Territories and Turkey.
