The UN has launched the largest-ever polio vaccination campaign in the Middle East, aiming to immunise more than 20 million children in seven countries amid an outbreak of the crippling virus in war-torn Syria, officials said Friday.

"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-immunised children wherever they are," Peter Crowley, who heads the UN children's agency's polio division, said in a statement.

The World Health Organisation last week confirmed the polio outbreak in Syria, which had been free of the disease since 1999.

The highly infectious disease affects mainly children under five and can cause paralysis in a matter of hours. Some cases can be fatal.

"In a region that had not seen polio for nearly a decade, in the last 12 months polio virus has been detected in sewage samples from Egypt, Israel, the West Bank and Gaza Strip," WHO and UNICEF said in a joint statement.

"It has so far left 10 children paralysed, and poses a risk of paralysis to hundreds of thousands of children across the region," they stressed, pointing out that Syria has seen its immunisation rate plummet from more than 90 percent before the conflict began in March 2011 to 68 percent today.

The whole region will now face an intense vaccination push over the next six months, and will be on heightened alert to spot cases that may have been missed, they said.

More than 650,000 children in Syria, including 116,000 in the strife-torn northeastern Deir Ezzor province where the polio outbreak was confirmed last week, had already received emergency vaccinations, UNICEF and the WHO said.

A new campaign aims to vaccinate 1.6 million children in Syria against polio, measles, mumps and rubella, while Jordan plans to immunise 3.5 million across the country, the UN agencies said, adding that some 18,800 kids had received vaccines in Jordan's Zaatari refugee camp in recent days.

In Iraq, a vaccination campaign has started in the west of the country, with another pending in the Kurdistan region, while Lebanon plans to launch a nationwide vaccination push this week and Turkey and Egypt by the middle of the month.

Thanks to a global drive against polio, the virus is now endemic in just three countries: Afghanistan, Pakistan and Nigeria.

"Preliminary evidence indicates that the polio virus is of Pakistani origin and is similar to the strain detected in Egypt, Israel, the West Bank and Gaza Strip," Friday's statement said.