LAGOS, Nigeria -- The World Health Organization has confirmed a third case of polio in an area of Nigeria newly liberated from Boko Haram Islamic extremists, the Rotary Club said Monday, amid fears the disease could resurge in neighbouring countries.

The West African nation that once was the global epicenter of the wild polio virus had been declared polio-free last year, along with the African continent. But two cases were discovered last month among refugees from areas recently won back by Nigeria's military from Boko Haram.

More cases are expected to be discovered in these areas. It is an indicator that Nigeria's war on the crippling disease cannot be won until it overcomes the insurgency by extremists who are violently opposed to Western medicine.

Rotary Club's field co-ordinator, Aminu Muhammad, told The Associated Press the new case, a 2-year-old boy, was found in Monguno local government area last month. The others were further south in Jere and Gwoza. All are in northeastern Borno state, where WHO says more than half of the health facilities are not functioning because of the fighting.

Rotary is part of a new emergency immunization drive that vaccinated more than 1.5 million children last week in Borno, where WHO has said the virus has been circulating undetected for five years and where Boko Haram began its Islamic uprising in 2009.

The campaign plans to reach 31.5 million children in northern Nigeria and 56.4 million across the country before the end of the year, according to the country's health ministry.

But the U.N. Children's Fund has warned that about 1 million children are in areas too dangerous to access.

The new cases "mean children across the Lake Chad region are now at particular risk," the director of polio eradication for UNICEF, Reza Hossaini, said last month.

The Lake Chad Basin area, where Boko Haram is active, is shared by Nigeria, Cameroon, Chad and Niger, some of the world's poorest states.

Nigeria's Health Minister Isaac Adewole also has warned of the risk, "given previous history of exportation to other countries and the suboptimal routine immunization coverage in several (African) countries."

Nigeria's military has helped with the vaccination drive, which included logistics and other aid from the U.S. Centers for Disease Control, the United Nations and Britain's Save the Children as well as government health workers. Military helicopters flew vaccines into places too dangerous to reach by road, and truckloads of troops and armoured cars escorted vaccinators elsewhere.

Muhammad said they were using "hit and run" tactics to reach kids in areas where Boko Haram is present.

Nigeria's military has said it has the insurgents "on the run" and needs only to clear them out of border areas and their northeastern stronghold in the Sambisa Forest. But a map used in the vaccination campaign shows almost all of Borno state is only "partially accessible," with four northern areas "inaccessible" and only the extreme south "accessible."

Boko Haram's uprising has forced some 2.6 million people from their homes in Nigeria and neighbouring countries.

There has been no major attack in months by the group, which is in the throes of a leadership struggle.

Polio remains in only two other countries, Pakistan and Afghanistan, also in areas affected by Islamic extremism.