Nigeria has reported the first two cases of polio after more than two years, in an area newly liberated from Islamic extremists who attacked polio vaccinators in the past, the government and the World Health Organization say.

Nigeria's removal from WHO's list of polio-endemic countries in October had meant the entire African continent was free of the crippling disease.

Two children have been paralyzed by polio in northeastern Borno state in two different local government areas that had been cut off by Boko Haram's Islamic extremist uprising, Health Minister Isaac Adewole said in a statement Thursday night.

"Our overriding priority right now is to rapidly boost immunity in the affected areas to ensure that no more children are affected by this terrible disease," he said.

He ordered the deployment of a national emergency response team. WHO said it was working with the government to urgently prevent more children from being paralyzed, with large-scale immunizations and other measures.

It was unclear how accessible the two areas are. The United Nations last month suspended aid to newly liberated but still dangerous areas of Borno after Boko Haram ambushed a humanitarian convoy, wounding three civilians including a UNICEF worker. That came even as aid groups declared half a million people are starving in those areas and children are dying daily of starvation.

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

The polio virus mainly affects children aged five and under. The highly contagious disease can invade the nervous system and paralyze susceptible children.