An Afghan child receives a polio vaccine during the anti-polio campaign in Ghazni province, eastern Afghanistan, April 18, 2016. (Xinhua/Sayed Mominzdah)

UNITED NATIONS, Aug. 12 (Xinhua) -- The World Health Organization (WHO) said Friday the confirmation of two new cases of polio in Nigeria was "a disappointing development," and the cases turned back eradication clock in the West African country.

Farhan Haq, the deputy UN spokesman, said at a daily news briefing here that the cases were discovered in parts of Borno in northeastern Nigeria that have recently become accessible, and that large areas of the state remained unreachable.

Africa was scheduled to be declared polio-free in about a year, Haq said, quoting WHO officials. "The clock will have to be turned back, ... but the goal remains within reach."

"A vaccination campaign will begin within the next 10 days," Haq said.

The Nigerian government on Thursday confirmed an outbreak of two cases of wild polio virus in Borno.

In 2012, Nigeria accounted for more than half of all polio cases worldwide, but the country has made significant strides in recent years, going two years without recording a single case.