In this photo taken on Saturday, Aug. 28, 2016, a child cries after she was administered with a polio vaccine during a house to house vaccination exercise in Maiduguri, Nigeria. An emergency polio vaccination campaign aimed at reaching 25 million children this year has begun in parts of Nigeria newly freed from Boko Haram Islamic extremists, with fears that many more cases of the crippling disease are likely to be found. (AP Photo/Sunday Alamba)

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 neighboring 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 coordinator, 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.

In this photo taken on Saturday, Aug. 27, 2016, a health official administers a polio vaccine to a child at a camp for people displaced by Islamist Extremist in Maiduguri, Nigeria. An emergency polio vaccination campaign aimed at reaching 25 million children this year has begun in parts of Nigeria newly freed from Boko Haram Islamic extremists, with fears that many more cases of the crippling disease are likely to be found. (AP Photo/Sunday Alamba)

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.

In this photo taken Saturday, Aug. 27, 2016, a polio campaigner smiles, during a polio vaccination excise in Maiduguri, Nigeria. An emergency polio vaccination campaign aimed at reaching 25 million children this year has begun in parts of Nigeria newly freed from Boko Haram Islamic extremists, with fears that many more cases of the crippling disease are likely to be found. (AP Photo/Sunday Alamba)

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 armored cars escorted vaccinators elsewhere.

In this photo taken on Sunday, Aug. 28, 2016, a health official writes a date on a wall of a house after administering a polio vaccine in Maiduguri, Nigeria. An emergency polio vaccination campaign aimed at reaching 25 million children this year has begun in parts of Nigeria newly freed from Boko Haram Islamic extremists, with fears that many more cases of the crippling disease are likely to be found. (AP Photo/Sunday Alamba)

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 neighboring countries.

In this photo taken on Sunday, Aug. 28, 2016 health officials administers polio vaccine to children at a camp for people displaced by Islamist Extremist in Maiduguri, Nigeria, Sunday Aug. 28, 2016. Health workers using military helicopters, all-terrain vehicles and tricycle taxis vaccinated hundreds of thousands of children this week, fighting the return of crippling polio in areas newly freed from Boko Haram extremists who violently oppose Western medicine. (AP Photo/Sunday Alamba)

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.

  • In this Sunday, Aug. 28, 2016 photo, a health official administers a polio vaccine to a child at a camp for people displaced by Islamist Extremists, in Maiduguri, Nigeria. An emergency polio vaccination campaign aimed at reaching 25 million children this year has begun in parts of Nigeria newly freed from Boko Haram Islamic extremists, with fears that many more cases of the crippling disease are likely to be found. (AP Photo/Sunday Alamba)

  • In this photo taken on Sunday, Aug. 28, 2016,h ealth officials wait for a briefing before administering polio vaccinations, at a camp for people displaced by Islamist Extremist in Maiduguri, Nigeria. An emergency polio vaccination campaign aimed at reaching 25 million children this year has begun in parts of Nigeria newly freed from Boko Haram Islamic extremists, with fears that many more cases of the crippling disease are likely to be found. (AP Photo/Sunday Alamba)