* Crippling virus confirmed in Deir al-Zor province
* UNICEF calls for vaccination of 500,000 Syrian children
* Seven Middle East nations plan to vaccinate 20 million
* Virus believed to have originated in Pakistan
Polio has broken out among young children in northeast Syria after probably originating in Pakistan and poses a threat to millions of children across the Middle East, the World Health Organisation (WHO) said on Tuesday.
The crippling disease, which is caused by a virus transmitted via contaminated food and water, could spread especially fast in Syria, where civil war has led to falling vaccination rates.
Twenty-two children in Deir al-Zor province bordering Iraq became paralysed on Oct. 17 and the polio virus has been confirmed in samples taken from 10 victims. Results on the other 12 are expected within days.
"This virus has come over land which means the virus is not just in that corner of Syria but in a broad area," Bruce Aylward, WHO assistant director-general for polio, emergencies and country collaboration, told Reuters in an interview.
"We know a polio virus from Pakistan was found in the sewage of Cairo in December. The same virus was found in Israel in April, also in the West Bank and Gaza.
"It... is putting the whole Middle East at risk quite frankly," he said by telephone from Oman.
Polio, which invades the nervous system and can cause irreversible paralysis within hours, can spread rapidly among children under five, especially in the unsanitary conditions endured by the displaced in Syria or crowded refugee camps in neighbouring countries.
It is endemic in just three countries - Nigeria, Pakistan and Afghanistan - raising the possibility that foreign fighters imported the virus into Syria, where Islamist militants are among the groups battling to oust President Bashar al-Assad.
Genetic sequencing of the virus found in Syria is expected within the next days, which will identify the geographic origin of the first polio outbreak in the war-torn country since 1999.
"Everything suggests this virus will be linked to the virus that originated in Pakistan," Aylward said.
"We are looking basically at re-infection of the Middle East. Syria is the canary in the coal mine," he said.
Most of the 22 Syrian victims are under two years old and are believed never to have been vaccinated or to have received only a single dose of the oral vaccine instead of the three which ensure protection, WHO spokesman Oliver Rosenbauer said.
"Immunisations have started in that area," Rosenbauer said, referring to Deir al-Zor, whose main city is partly controlled by Assad forces while rebels hold the surrounding countryside.
A previously planned immunisation campaign was launched in Syria on Oct. 24 to vaccinate 1.6 million children against polio, measles, mumps and rubella, in both government-controlled and contested areas, the WHO said on Tuesday.
Anthony Lake, executive director of the U.N. Children's Fund (UNICEF), said he had held "businesslike and encouraging" talks with Syrian Prime Minister Wael al-Halqi in Damascus.
He called for the estimated half a million Syrian children who have not been vaccinated against polio and other debilitating diseases because of the war to be vaccinated.
Syria has about three million under-5s in total.
As well as Syria, Aylward said at least six other Middle East nations - Egypt, Iraq, Turkey, Lebanon, Jordan and the Palestinian territories - plan polio immunisation campaigns.
"This will cover over 20 million children in the next months," he said.
The campaigns are likely to begin in early November and to last at least six to eight months, the WHO said in a statement.
About 4,000 refugees flee the war in Syria every day, mainly crossing into Lebanon, Iraq, Jordan and Turkey.
Before the conflict, which began with peaceful protests in March 2011 and led to a civil war, 91 percent of Syrian children had been vaccinated against diseases including polio, but the rate has fallen to about 68 percent, Rosenbauer said.
"So it makes sense that very young kids would get it."