A suicide bomber blew himself up close to a police van in northwest Pakistan on Tuesday, killing six people and wounding 17 others, police said.

The blast took place in Bannu close to the Afghan border, said officer Nawaz Khan.

He said initial reports suggested the attacker was on foot.

Police officer Iftikar Khan said one of the six victims was a police officer, while the rest were civilians.

A spokesman for the Pakistani Taliban claimed responsibility.

Islamist militants seeking to topple Pakistan's U.S.-allied government have carried out scores of attacks across the country, killing thousands, over the last four years. The military has launched offensives in the northwest where the insurgents are based, but the attacks have continued.

Police arrested four suspected Pakistani Taliban militants Tuesday while they were riding in a taxi in the southern city of Karachi, said police officer Omar Shahid. The arrests were made after police received a tip, he said.

The men were allegedly planning to attack police offices, government buildings and other targets in the city, said Shahid. Police confiscated three suicide jackets containing explosives and several guns from their hideout, he said.