ISLAMABAD: Some 240,000 children have missed UN-backed vaccinations against polio because of security concerns in Pakistan's tribal regions bordering Afghanistan, a top official with the World Health Organization said Friday.

Dr. Nima Saeed Abid, the acting WHO chief in Pakistan, said health workers have not been able to immunize children in the North and South Waziristan regions – strongholds for Taliban militants – since July 2012.

Pakistan is one of the few remaining countries, along with Afghanistan and Nigeria, where polio is rampant. As many as 58 cases were reported in Pakistan in 2012, down from 198 in 2011.

She said polio transmission is now concentrated in core endemic areas – central Khyber Pakhtunkhwa province, the Federally Administered Tribal Regions or FATA, parts of Karachi, Quetta and nearby Killa Abdullah and Pishin districts in south western Baluchistan province.

Abid said that 15 health workers have been killed in the anti-polio campaign in Pakistan since July 2012.

Correction: A previous version of this report incorrectly stated WHO as UNICEF. The error has been corrected.