Geneva: UN officials have begun mobilising to vaccinate 2.5 million young children in Syria and more than 8 million others in the region to combat what they fear could be an explosive outbreak of polio, the incurable viral disease that cripples and kills, which has reappeared in the war-ravaged country for the first time in more than a dozen years.

The officials said the discovery a few weeks ago of a cluster of paralysed young children in Deir Al Zor, a heavily contested city in eastern Syria, had prompted their alarm, and that tests conducted by both the government and rebel sides strongly suggested the children had been afflicted with polio.

The possibility of a polio epidemic in Syria, where the once-vaunted public health system has collapsed after 31 months of political upheaval and war, came as the United Nations is increasingly struggling with the problem of how to deliver basic emergency aid to millions of deprived civilians there.

Valerie Amos, the top UN relief official, told the Security Council on Friday that combatants on both sides of the conflict had essentially ignored the council’s October 2 directive that they must give humanitarian workers access to all areas in need.

Speaking to reporters afterward, Amos said she had expressed to council members “my deep disappointment that the progress that we had hoped to see on the ground as a result of that statement has not happened, and in fact what we are seeing is a deepening of the crisis”.

Bruce Aylward, the assistant director general for polio and emergencies at the World Health Organisation, which is helping to lead the new polio vaccination effort in Syria, said officials at the agency were taking no chances and assuming that the 20 paralysed children in Deir Al Zor were polio victims.

“This is polio until proven otherwise,” he said in a telephone interview from the group’s headquarters in Geneva.

Despite the war, Aylward said, he believed both sides understood the urgent need for repeated vaccinations of all young children because polio can spread indiscriminately and is so difficult to eradicate. Nonetheless, he said, it remained unclear whether the vaccination effort in all parts of Syria would be impeded by the conflict’s chaos and politics.

“The virus is the kind of virus that finds vulnerable populations, and the combination of vulnerability and low immunisation coverage, that is a time bomb,” he said. “There is a real risk of this exploding into an outbreak with hundreds of cases.”

The WHO, working with the UN Children’s Fund and other aid groups, has organised a plan to administer repeated oral doses of polio vaccine in concentric geographical circles, starting with children in Deir Al Zor and eventually reaching western Iraq, southern Turkey, Jordan, Israel, the Palestinian territories and Egypt. In Lebanon, home to more than 700,000 Syrian refugees, public health officials said Friday that they were undertaking a related effort to vaccinate all children younger than 5.

Altogether, Aylward said, more than 10 million young children in the Middle East would get polio vaccinations over the next several weeks.

The WHO has spent 25 years trying to eradicate polio. In recent years its presence had narrowed to just three countries - Nigeria, Pakistan and Afghanistan - from more than 125 when the campaign began in 1988.

The virus is highly infectious and mainly affects children younger than 5. Within hours it can cause irreversible paralysis or even death if breathing muscles are immobilised. The only effective treatment is prevention, the WHO says on its website, through multiple doses of a vaccine.

While the source of the Syrian polio strain remained unclear, public health experts said the jihadists who had entered Syria to fight the government of President Bashar Assad may have been carriers.

Aylward said there were some indications that the strain had originated in Pakistan. He cited the recent discovery of the Pakistani strain in sewage in Egypt, Israel, the West Bank and Gaza.

The Syrian aid crisis portrayed by Amos in her Security Council briefing reflected new levels of frustration over the council’s inability to act decisively on the conflict, despite its binding - and so-far successful - September 27 resolution on the dismantling of Syria’s chemical weapons arsenal.

By contrast, the council’s October 2 statement requesting that all combatants in Syria protect civilians and allow unfettered access for humanitarian aid has no enforcement power.

“This is a race against time,” Amos said. “Three weeks have passed since the adoption of the council’s statement, with little change to report.”

Amos told the council that the Syrian government had withheld approval of more than 100 visas for UN staff members and members of other international aid groups, and had restricted workers from operating in areas with the greatest need.

She also said as many as 2,000 armed opposition groups in Syria had made travel within the country increasingly dangerous. Kidnappings of humanitarian workers are increasingly common, she said, citing an instance last week when “we had a convoy that was ready to go, but we could not get enough drivers as they fear for their lives.”