Witnesses said three men drove up to the police station in Karak area near Peshawar in a pick-up truck.
After two men got out of the vehicle, the driver rammed the truck packed with explosives into the police station.
District police chief Sajid Mohmand said two policemen and one civilian were killed in the attack. Fourteen policemen were among the 26 injured.
The blast, which occurred at the main gate of the police station, also damaged a nearby mosque. The injured were taken to a nearby hospital and officials said some of them were in a critical condition. They said the number of dead could rise.
Security personnel cordoned off the area after the blast and launched a search for the suicide bomber's two accomplices.
Authorities also imposed curfew in Karak. No group claimed responsibility for the attack but Taliban militants have targeted police, security forces and government offices in northwest Pakistan over the past three years.
Though Karak district lies between the militancy-affected areas of Lakki Marwat, Bannu and Kohat, this is the first time a suicide attack was carried out in the area. Police officials said they had prevented another suicide attack on a police station at Mattani, a suburb of Peshawar, by intercepting the bomber as he tried to enter the building.
There were also reports that four suicide attackers had entered Mingora, the main city in the restive Swat district of North West Frontier Province.
Earlier, a police officer was killed and several others were injured when militants carried out near-simultaneous suicide attacks on two police stations in NWFP last Saturday.
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