The World Health Organization says it has confirmed 10 cases of polio among young children in northeastern Syria, and expressed concern the disease could spread inside and outside the country.
The U.N. health agency said Tuesday that results from 12 other children with polio symptoms now being examined are expected within days.
UNICEF Executive Director Anthony Lake told the Associated Press that while in Damascus he urged the warring sides to grant health workers access to 500,000 children who have never received immunization.
Polio is a highly infections disease that can be transmitted through food and water and spreads rapidly among children under five. Syria has not had an outbreak of polio since 1999, but the WHO warned earlier this month that the country was at high risk because of its ongoing civil war.
More than two years of fighting in Syria has strained basic services in some areas and forced more than a quarter of the country's population to leave their homes.
The unsanitary conditions endured by the displaced in Syria or crowded refugee camps in neighboring countries exacerbate the problem.
Meanwhile, U.N.-Arab League envoy Lakhdar Brahimi said it was up to Syrians to shape their political transition in hoped-for peace talks, after warning of the potential "Somalisation" of the country.
But the talks have been cast into doubt by the increasingly divided opposition's refusal to attend unless the departure of President Bashar al-Assad is on the table, a demand rejected by Damascus.
Representatives of Syria's domestic opposition groups also said they would not participate in peace talks under the umbrella of the Syrian National Coalition, the opposition's body in exile.
"It is not possible to unite the opposition and he has to find another way. There is no doubt we will not go under the umbrella of the coalition because we differ with them completely. There is a difference between our agenda and theirs."
Syria's political opposition in exile is facing mounting pressure from fighters on the ground to reject any negotiations that would not require Mr. Assad's ouster.
Also Tuesday, Syrian state media said Deputy Prime Minister Qadri Jamil was dismissed from his post due to government complaints about his performance and lack of coordination with other officials.
Jamil had been in Geneva recently and met U.S. officials there, according to U.S. sources in the city. It is unclear if his dismissal was linked to the meetings.