Pakistan, Cameroon and Syria pose the greatest risk of exporting the virus to other countries, the Geneva-based WHO said in an e-mailed statement today. Those nations should declare national public health emergencies and ensure that residents have been vaccinated before they travel internationally, WHO Director General Margaret Chan said, citing recommendations from an emergency committee.

Polio, driven to the brink of eradication in 2012, has resurged as conflicts from Sudan to Pakistan disrupt vaccination campaigns. The number of cases reached a record low of 223 globally in 2012 and jumped to 417 last year, according to the WHO. There have been 68 this year as of April 30, during what is usually polio’s “low season,” the United Nations health agency said.

“Conflict makes it very difficult for the vaccinators to get to the children who need vaccine,” said David Heymann, a professor of infectious diseases at the London School of Hygiene and Tropical Medicine, in an interview before the WHO’s announcement. “It’s been more difficult to finish than had been hoped.”