A senior health official has been killed and three guards injured in a bomb blast in Pakistan's tribal region bordering Afghanistan, officials say.
The dead man, Abdul Ghani Khan, played a key role in a polio immunisation drive in the Bajaur tribal region.
Dr Khan was returning from a meeting of tribal elders to persuade them to end their opposition to the campaign.
It is not clear if he was targeted because of his work to eradicate polio in the area.
No one has admitted to carrying out the blast. Officials said the assailants used a remote-controlled bomb.
The government is facing resistance from some tribes in its campaign to vaccinate children against polio.
The border areas are thought to contain strong Taleban support
Some tribal leaders say the vaccine is a part of a US conspiracy to reduce fertility and reproduction rates.
Two of the three guards travelling with Dr Khan are in a serious condition after the blast, in a village around 50kms (30 miles) northeast of Khar, the main town in the Bajaur tribal region.
Pro-Taleban militants are known to be active in the area.
Pakistan is one of the few countries in the world where polio remains endemic.
It is a highly infectious viral disease which mostly affects children under five-years-old.
The virus attacks the central nervous system, causing paralysis, muscular atrophy and deformation. It can ultimately lead to death.
Pakistan last year confirmed 40 cases of the crippling disease, the World Health Organisation (WHO) said.
Elsewhere, police in the southern city of Karachi say that they have arrested three suspected suicide bombers linked to al-Qaeda following a shoot-out.
They say that they have recovered a jacket for use in suicide attacks, hand grenades, pistols and information on how to prepare for suicide bombings.
They say the three suspects had been trained in the restive tribal regions of South Waziristan.
There have been a number of suicide attacks in Pakistan in recent weeks.