The World Health Organization says it's aiming to vaccinate more than 115 million children against polio across Africa next week, in its continuing bid to wipe out the crippling disease.

In a statement Thursday, the U.N. health agency said all children under five are being targeted by more than 190,000 vaccinators in 13 central and West African countries, including Nigeria, Congo and the Central African Republic.

Eradicating requires reaching more than 90 percent of vulnerable children and rigorous surveillance, tasks that have proven nearly impossible in war-torn areas. Last year, WHO identified several polio cases in northern Nigeria despite previously declaring the country "polio-free."

Nigeria is thought to be the only country in Africa where polio is spreading. Elsewhere, polio continues to sicken children in Pakistan and Afghanistan.