At least seven persons, including four policemen, were killed and around 40 others injured in northwest Pakistan on Sunday in a terror strike on a checkpost and a subsequent suicide blast by a burqa-clad woman at a hospital where the victims of the first attack were shifted.

Both the attacks took place in Dera Ismail Khan district of Khyber Pakhtunkhwa, a day after the first-ever successful election was held in the newly merged tribal areas of the province.

Four unidentified armed men riding two motor-bikes opened fire on two policemen, who were on duty at the Kotla Syedan checkpost, and killed them, District police chief Saleem Riaz told the media.

When the bodies of the slain policemen were brought to the district hospital, a burqa-clad suicide bomber, already sitting there, targeted the people who gathered around the ambulance, the official said.

"Five people, including two policemen of the force, were killed in the suicide blast," Riaz said.

He said the suicide bombing by a female was first ever and unexpected incident in the region.

According to eye witnesses, the bomber was wearing a head-to-toe veil. Police also recovered hairs and feet of the attacker and sent them for forensic examination.

The Tehreek-e-Taliban Pakistan (TTP) claimed responsibility for the attack.

The seriously injured people were shifted to the Combine Military Hospital for medical cover. The police and civil administration reached the spot immediately after the explosion.

Riaz said that 7 to 8 kg explosive was used in the suicide explosion. He confirmed seven fatalities, in both the terror act.

The death toll could rise as the majority of the victims were in critical condition, he said.

He said that security check is done by the police on the entry of people into the hospital. But due to tradition and customs of the area females are exempted. He said the suicide bombing by a female was first ever and unexpected incident in the region.

Dera Ismail Khan is considered as gateway to North Waziristan and South Waziristan district which were once hotbed of militants.