* Seven countries, regions involved in mass immunisation
* "Rush to vaccinate" in stricken area of Syria
* Crippling viral disease spreads rapidly among children
* One infected child can lead to another 200 polio cases (Adds quotes)
By Stephanie Nebehay and Kate Kelland
GENEVA/LONDON, Nov 8 (Reuters) - More than 20 million children are to be vaccinated in Syria and neighbouring countries against polio to try to stop the spread of the crippling infectious disease following its re-emergence there 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 Organisation (WHO) and the U.N. Children's Fund (UNICEF) said.
Aiming to repeatedly vaccinate about 20 million children in seven countries and territories, it will be the largest-ever consolidated immunisation response in the Middle East. Civil war in Syria and the movement of refugees in the region have hampered immunisation in recent years.
"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, UNICEF's Chief of Polio, said in a statement.
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 told a briefing in Geneva.
The first polio outbreak in Syria since 1999, it has so far paralysed 10 children and poses a risk of paralysis to hundreds of thousands of children across the region, the WHO said.
Emergency immunisation campaigns in and around Syria to prevent transmission of polio and other preventable diseases have vaccinated more than 650,000 children in Syria, including 116,000 in the northeast Deir al-Zor province where the polio outbreak was confirmed. There is high demand among parents.
"There's a rush to vaccinate," Bari quoted a WHO official present in Deir al-Zor during vaccinations there as saying.
The WHO was "optimistic" about containing the outbreak, Bari said. "That said, we're certainly going to see more cases."
Syria's immunisation rates have plummeted from more than 90 percent before the conflict to currently around 68 percent.
Preliminary evidence has indicated the virus is of Pakistani origin, but results of genetic sequencing are awaited. Polio is still endemic in Pakistan, along with Nigeria and Afghanistan.
"We're never going to know exactly how it arrived in Syria," Bari said.
"We also know that adults tend to have much higher level of immunity already developed, so it is unlikely that adults brought this. It is more likely some other route."
Children in unsanitary conditions are particularly vulnerable to infection with the polio virus, which spreads through faecal-oral transmission and contaminated food and water.
It attacks the nerves and can kill or paralyse, often spreading widely and unnoticed before it starts crippling children. For every case of polio, 200 children can be infected. There is no cure, but it can be prevented though vaccination.
The WHO said the vaccinations were vital in a region that had not seen polio for nearly a decade, but which in the last 12 months has detected the polio virus in sewage samples from Egypt, Israel, the West Bank and Gaza Strip. (Writing by Kate Kelland, editing by Ralph Boulton)