Efforts to vaccinate 165,000 children against polio in Sudan have been blocked by the government and rebels, the UN humanitarian chief says.
John Ging said the two sides should stop "filibustering" and give health workers access to children in the South Kordofan and Blue Nile states.
The government and rebels had ignored a Security Council resolution to give health workers access, he said.
The UN fears that conflict in the two states could lead to a polio outbreak.
According to the UN World Health Organization, Sudan has been polio-free for more than two years.