Ongoing violence in Syria has hindered an international campaign to administer polio vaccines to thousands of children, just months after experts confirmed the country's first polio cases in more than a decade. United Nations aid groups announced yesterday that fighting in the northeast province of Raqqa has brought the vaccination campaign to a halt, putting 100,000 children at risk of infection. As Reuters reports, the groups are appealing for access to the region, which is the only province under the control of armed rebel groups.

"We haven't reached Raqqa town in this second round of immunization," Elizabeth Hoff of the World Health Organization (WHO) told Reuters. "There are approximately 100,000 children out of reach in the town and its outskirts."

In October, officials confirmed that 17 children had been infected by the disease, which can leave younger patients paralyzed or, in some cases, dead. The outbreak was Syria's first since 1999, and it prompted a massive campaign to vaccinate 2 million children each month until May. But getting access to certain parts of the war-torn country has proven difficult, due to ongoing unrest in a nearly three-year civil war that has killed at least 100,000 people.

The WHO and other aid workers were able to vaccinate an estimated 2.15 million children during the second round of immunization last week, including some in Raqqa province. In areas where they've been granted access, the response has been largely encouraging.

"The information campaign has been very strong, parents are bringing their children," Hoff said. "The uptake is very good."

It's not clear whether Syrian authorities or rebel groups will allow freer access for aid workers, though US Secretary of State John Kerry says it could be included as a provision in a future ceasefire agreement. Over the weekend, Kerry met with Russian Foreign Minister Sergei Lavrov and a UN mediator to discuss possible diplomatic solutions ahead of planned peace talks to be held in Switzerland next week.