GENEVA - Nigeria has reported that two children in Borno state have been paralyzed by polio, the first cases of the disease in the country in two years, the World Health Organization (WHO) said on Thursday.

Nigeria and the U.N. agency are conducting immunization campaigns and strengthening surveillance systems that help catch the virus early, the WHO said in a statement on the latest setback to its global program to eradicate polio.

“The overriding priority now is to rapidly immunise all children around the affected area and ensure that no other children succumb to this terrible disease,” said Dr. Matshidiso Moeti, WHO Regional Director for Africa.

The Nigerian Health Minister, professor Isaac Adewole, called the development a setback, but said, “I can assure the nation that we will do everything possible to be on top of the situation,” according the Leadership newspaper. The government is expected to release a further statement on the outbreak later Thursday.

The cases reportedly occurred in a state in Nigeria’s north, where the insurgent group Boko Haram has been active. Last month, Doctors Without Borders issued a statement calling the health situation in Borno State “critical” with at least 500,000 people in urgent need of drinking water, food, medical care and shelter. The group called for a “massive relief operation” as the Nigerian army retook control of the main towns and villages, some of which had been under Boko Haram’s sway for two years.

Pakistan and Afghanistan have also reported polio this year, with 21 cases to date, WHO said.