Military helicopters on Friday rushed polio vaccines to dangerous parts of northeastern Nigeria, where Boko Haram operates and two paralysed toddlers were recently discovered, a Nigerian health official said.

The World Health Organization said the new cases indicate the wild polio virus has circulated undetected for five years in Borno state – a major setback after Nigeria was declared polio-free in October.

Recent attacks by the Islamic extremists prevented the emergency operation from getting health workers to two parts of Borno State where the children were found, Borno state health commissioner Ibrahim Miringa told The Associated Press.

Massive challenges face Nigerian health workers supported by the World Health Organization and the U.S. Centers for Disease Control to urgently vaccinate millions of endangered children in an area where the U.N. suspended aid after a military-escorted humanitarian convoy was attacked last month.

“Not all the areas that have been liberated by the military could be accessed by our health officials because of recent attacks in Jere and Gwoza local government areas carried out by Boko Haram,” Miringa said. Jere and Gwoza are the areas where the first two cases of polio were found in Nigeria in more than two years, the WHO announced Thursday night.

Nigeria's removal from WHO’s list of polio-endemic countries had meant all of Africa was free from the crippling disease. Only two other countries remain on the list – Afghanistan and Pakistan.

Miringa said the two children, aged under 2, were among refugees arrived from areas newly freed from Boko Haram.

He said health workers have been vaccinating refugees as they arrive in major camps. “But recent liberation of internally displaced persons from the hinterlands and their being camped in satellite camps have made the immunisation exercise difficult.”

International organisations plan to support Nigerian health workers in using “a hit-and-run kind of strategy,” the WHO director for polio eradication, Dr. Michel Zaffran, told reporters in a conference call.

A massive vaccination drive was starting in Borno and within weeks would be carried to nearby Benin, Chad, Central African Republic, Chad and Cameroon, Zaffran said. (AP)

•Photo shows a Nigerian military helicopter in flight.