A top UNICEF official says as many as 240,000 children have missed UN-backed vaccinations against polio because of security concerns in Pakistan's tribal regions bordering Afghanistan, The Associated Press reports.

Dr. Nima Saeed Abid, acting UNICEF chief for the polio eradication program in Pakistan, says health workers have not been able to immunize children in the North and South Waziristan regions — Taliban strongholds — since July 2012.

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

Abid said Friday, March 29, that 15 health workers have been killed in the anti-polio campaign in Pakistan since July 2012.

Pakistani militant groups oppose the vaccinations and accuse the workers of spying for Washington.