At least 17 children in eastern Syria have been paralyzed from a recently confirmed outbreak of polio, the World Health Organization said Tuesday, punctuating the health risks to a population ravaged by more than six years of war.

It is the second outbreak of the crippling disease to hit Syria since the war began, and largely reflected the inability of health workers to immunize all children caught in conflict zones where access is difficult and sanitation is poor.

The polio virus, once thought to verge on eradication, is one of the most contagious diseases in inadequately protected areas. One confirmed case of paralysis is considered an outbreak, as doctors assume it means up to 200 other people may have been exposed to the virus.

Tarik Jasarevic, a spokesman for the World Health Organization in Geneva, said there was an urgent need to vaccinate more than 400,000 children under the age of 5 in the Deir al-Zour area, where the outbreak was first confirmed in an announcement made by the organization on June 8. He described the outbreak as “very serious.”