Three Pakistani security forces have been killed in a bomb explosion near a military convoy in the tribal region of north-western Pakistan, the army said.
The blast happened near Khair, the main town in the Bajaur tribal area on the border with Afghanistan.
The dead included a senior paramilitary officer, officials said.
The incident comes ahead of Pakistan's general election on Monday. Attacks by pro-Taleban militants in the north-west have soared in the past year.
Army spokesman Major General Athar Abbas told the Associated Press news agency that the explosion struck a military convoy which was heading from a post near the Afghan border to Khar.
No one has claimed responsibility for the incident yet.
Bajaur was the scene of two major attacks against suspected militants in 2006, which caused great anger among the locals and people elsewhere in Pakistan.
In January 2006, at least 13 people were killed when American forces struck three compounds, apparently after reports that al-Qaeda number two Ayman al-Zawahri was in the area.
An air raid by Pakistani forces on a religious school in October left more then 80 people dead.
Hundreds of Pakistani troops have been killed in clashes with militants since they were sent into tribal areas bordering Afghanistan after the attacks of 11 September, 2001.
The US and Nato say the border region has become a "safe haven" for al-Qaeda and the Taleban.