A man was walking close to a police van, thought to be the target, when he detonated the device, local police said.
Around 20 people were wounded by the bomb in Bannu town, which is within Khyber Pakhtunkhwa province, previously called the North-West Frontier province and which borders Afghanistan.
"The death toll is now six. They include two boys aged seven and nine, one police official and three pedestrians," police chief Iftikhar Khan said.
The area is a noted stronghold of the Taliban and al-Qaida extremist groups operating within Khyber Pakhtunkhwa province. The Khyber Pass is important to the groups because it links the province to Afghanistan and is a major supply route.
The area also has the Kohalla Bridge in Circle Bakote, a major crossing point over the Jhelum River in the east. The bridge is a link on routes toward the frontier with the disputed Indian state of Kashmir, where, according to Indian administration authorities, Pakistani-based militant groups also operate.
Pakistani police and security authorities, who attempt to disrupt supply lines, often are targets of militant groups within the province.
In February, a suicide bomber attacked a vehicle carrying tribal police in Jamrud town, near Pakistan's border with Afghanistan, killing 17 people, including 10 policemen.
A witness at the time said the suicide bomber was able to get inside the police vehicle before detonating his bomb.
"I saw him running and jumping into the Khasadar (police) vehicle and then an explosion pushed me down to the ground some 50 yards away," local tribesman Izzakhana Afridi said.
Jamrud is within the Khyber Agency, one of the Federally Administered Tribal Areas of Pakistan, and is at the start of the Khyber Pass. The town has road and rail links with the Pakistani city of Peshawar and with Landi Kotal, located near the border with Afghanistan.
The Jamrud attack came a day after Pakistani security officials claimed they recently had killed Taliban chief Hakimullah Mehsud, 28. They said he died from wounds inflicted during a U.S. drone missile attack operation in the northwest Waziristan area.
However, in April doubts were cast upon the veracity of the claims after two videos emerged that appeared to show him alive.
In one of the videos, Mehsud referred to reports of his death as propaganda and threatened attacks on major U.S. cities.
Another of the videos has Mehsud beside a map of the United States with explosions marked in three locations, but not clearly enough outlined to pinpoint exact targets.
Pakistani intelligence officials later said they believed Mehsud was only wounded, but they claimed his authority within the Taliban administration had been diminished.
Police in New York were quick to dismiss any connection between Mehsud and the failed attempt to detonate a car bomb in the city's Times Square.