Over 370,000 children are expected to miss out on the fresh polio vaccination drive in its restive tribal belt in Pakistan due to security problems.
At the start of May the World Health Organization (WHO) declared a global "public health emergency" after new polio cases began surfacing and spreading across borders from countries including Pakistan.
Pakistan's seven semi-autonomous tribal areas along the Afghan border are the epicentre of the country's polio cases and the government has set up checkpoints to ensure anyone leaving the belt is immunised.
A three-day vaccination drive began on Monday in four tribal areas, a senior government official in Peshawar, the region's main city, told reporters, with more than 620,000 youngsters on course to receive polio drops.
But the official, speaking on condition of anonymity, warned that children in three of the four targeted areas "would not be able to receive polio drops because of the militancy and opposition to the immunisation".
Violence has badly hampered the campaign to stamp out polio in Pakistan, where militant groups with strongholds in tribal areas, including the Pakistani Taliban, see vaccination campaigns as a cover for espionage.
"A total of 369,039 in three districts would not receive polio drops because of the law and order situation," the official said.
The three districts in question are North Waziristan, South Waziristan and Mohmand, the official said. Part of the fourth district, Khyber, would also be affected.
Pakistan is one of only three countries where the crippling disease remains endemic, and is responsible for 80 percent of polio cases diagnosed around the world this year.
Some 56 people including health workers and police officials providing security have been killed in militant attacks on polio vaccination teams in Pakistan since December 2012.
There are also long-running rumours about polio drops causing infertility.
Two senior health officials confirmed the latest drive and figures and told AFP that WHO and UNICEF was helping and providing logistic support in the polio campaign.