A polio vaccination programme in a remote Pakistani tribal region has been suspended after villagers threatened health workers, officials say.
Hardline clerics in the area are against the programme, saying it is a US conspiracy to render people incapable of producing children.
Officials say that up to 4,000 children in two villages in the Bajaur tribal region were due to be vaccinated.
Pakistan is one of only five countries where the polio virus still exists.
Eleven new cases have been reported so far this year.
Hotbed
"We have stopped vaccination programme after tribesmen threatened our workers and broke their equipment in Sarkari Killa and Kotgi Charmang villages on Tuesday," Dr Cherag Hussain told the Reuters news agency.
"They have threatened to kill health workers if they visit again."
On Tuesday officials said that armed men abducted and beat 11 health workers sent to Bajaur to administer polio vaccinations.
They said that health workers were held for four hours as their captors smashed vaccination kits.
Dr Hussain said that the work in Bajaur was part of a national drive this year to immunise 32 million children aged under five-years-old.
The campaign in the Bajaur region - part of North West Frontier Province (NWFP) - was also suspended early this year after a doctor and a health worker were killed in a roadside blast.
Correspondents say that Bajaur is considered a hotbed of support for Islamic militants.
Health officials in the area have been trying to dispel rumours - sometimes spread by radio stations and mosques - that the polio campaign is a Western conspiracy to reduce Muslim populations.
The disease has been eliminated in developed nations but persists in parts of India, Nigeria, Afghanistan and Pakistan.
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