United Nations officials said Friday that they were mobilizing to vaccinate 2.5 million young children in Syria and more than eight 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 that the discovery a few weeks ago of a cluster of paralyzed young children in Deir al-Zour, a heavily contested city in eastern Syria, had prompted their alarm, and that tests conducted by both the government and rebel sides strongly suggested that 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 relief official at the United Nations, told the Security Council on Friday that combatants on both sides of the conflict had essentially ignored the Council’s Oct. 2 directive that they must give humanitarian workers access to all areas in need.