A bomb blast targeting police killed five people in northwest Pakistan, rescue and police officials have said.

The bomb exploded close to a police patrol in Dera Ismail Khan city, police official Mohammad Adnan said on Friday.

"Five people have been killed and over 20 others injured," Bilal Faizi, spokesperson for Rescue 1122, told Anadolu over the phone. The injured, including several in critical condition, have been rushed to a local hospital, he added.

Rescue official Aizaz Mehmood said five people were killed and another 21 suffered wounds, according to Reuters news agency.

The city of Dera Ismail Khan, where the blast took place, lies on the edge of lawless tribal districts bordering Afghanistan that have long been home to fighters both domestic and foreign.

The bomb exploded close to the route of a police patrol in the city, said police official Mohammad Adnan, adding that it was not immediately clear if the incident was the result of a suicide attack or a bomb planted nearby.