Two polio vaccination workers have been killed in a landmine blast in the Kurram tribal region of north-west Pakistan, officials say.
The men were visiting a village in the Malikhel area when the mine went off. No group has claimed the attack.
Correspondents say Sunni militants may have planted the mine - the upper Kurram area is dominated by Shias who do not oppose anti-polio efforts.
At least 11 polio workers have now been killed in Pakistan in the past month.
Seven other charity workers were killed in January in the Swabi area - it is not clear if they were targeted because their charity offered vaccinations or education for girls.
The Taliban have threatened anti-polio efforts across Pakistan - accusing health workers of working as US spies and alleging that the vaccine makes children sterile.
Along with Afghanistan and Nigeria, Pakistan is one of only three countries where polio is still endemic.
Jawad Ali, in charge of the anti-polio campaign in Kurram, told the AFP news agency that the pair had been killed "on the spot".
He said the team had been visiting areas not covered by a three-day vaccination campaign that ended on Wednesday.The BBC's M Ilyas Khan in Islamabad says it is unlikely the polio workers were targeted deliberately.
The upper Kurram valley is inhabited by the Shia Turi tribe - which traditionally abhors the Sunni Taliban - it is also an area blighted by sectarian violence, he says.
Landmines are frequently planted in these areas to cause Shia casualties or hinder their free movement, our correspondent says.
Kurram is part of Pakistan's semi-autonomous tribal region near the Afghan border where Taliban and al-Qaeda-linked militants have strongholds from which they mount attacks on Pakistani, Afghan and Western targets.