Displaced Syrian children line up to receive polio vaccinations at a refugee camp in the southern port city of Sidon, Lebanon. AP Photo/Mohammed Zaatari

More than 20 million children are set to be vaccinated against polio in Syria and neighboring countries to try stopping the spread of the paralyzing infectious disease following its re-emergence in Syria after 14 years, United Nations agencies said on Friday.

The mass vaccination against polio, which can spread rapidly among children, is already under way in the Middle East a week after the region declared a polio emergency, the World Health Organization (WHO) and UNICEF said.

The threat of the re-emergence of polio is not limited to countries near Syria, a paper released in the Lancet medical journal on Friday 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.

The U.N.'s aim to vaccinate about 20 million children in seven countries and territories will be the largest-ever consolidated immunization drive in the Middle East.

"The polio outbreak in Syria is not just a tragedy for children, it is an urgent alarm – and a crucial opportunity to reach all under-immunized children wherever they are," Peter Crowley, UNICEF's chief of polio, said in a news release.

He said the recent outbreak in Syria, confirmed by the WHO last week, should "serve as a stark reminder to countries and communities that polio anywhere is a threat to children everywhere."

WHO spokeswoman Sona Bari said it would take six months of repeated campaigns to reach 22 million children.

"It is going to need quite an intense period of activity to raise the immunity in a region that has really been ravaged both by conflict in some parts, but also by large population movements," she said at a briefing in Geneva.

The first polio outbreak in Syria since 1999 has so far paralyzed 10 children and poses a risk of paralysis to hundreds of thousands of others across the region, the WHO said.

Syria's immunization rates have plummeted from more than 90 percent before the conflict to around 68 percent now.

Preliminary evidence has indicated the virus is of Pakistani origin, but results of genetic sequencing are still awaited. Polio is endemic in Pakistan, along with Nigeria and Afghanistan.