A bomb blast on a bus in Peshawar killed at least nine people on Saturday, officials said, in the latest attack to hit Pakistan's troubled northwest ahead of historic polls next month.

The explosion occurred just hours after militants blew up the election offices of an independent candidate in the North Waziristan tribal district, fuelling concerns that violence will mar general elections on May 11.

"At least nine passengers have been killed and seven injured. Bomb disposal officials told me that it was a timed device," senior police official Fazal Wahid told AFP.

Another officer, Imran Shahid, said police were investigating the possibility a suicide bomber was involved in the attack which occurred as the bus was passing through the city's Matani suburb.

There was no immediate claim for responsibility, but Peshawar is regularly targeted by the Pakistani Taliban who have waged an insurgency against the state since 2007.

An intelligence official in the city said the attack may be a reaction to a fresh military push in the Tirah valley of the Khyber tribal district, where the army has been fighting Taliban and Lashkar-e-Islam militants.

Military officials said heavy fighting between Pakistani troops and militants has killed 23 soldiers and 110 militants in Khyber this week.

A senior military official late Saturday told AFP that seven more militants were killed during the last 24 hours in Khyber.

"In the ongoing military operation in Tirah, Khyber, two important hideouts of miscreants have been busted and militants' bases in Sipah area have also been captured," the official added.

Khyber straddles the NATO supply line into Afghanistan, used by US-led troops to evacuate military equipment ahead of their 2014 withdrawal.

Officials say securing Khyber is key for protecting security in Peshawar, ahead of elections which will mark the country's first democratic transition of power after a civilian government has served a full term in office.

Senior bomb disposal expert Abdul Haq told AFP that four to five kilograms (nine to 11 pounds) of highly explosive material was used.

The bomb destroyed three shops and a motorcycle, police and witnesses said.

"I was going to buy some milk when a huge blast took place. It was so powerful that it threw me back in my shop," Asad Khan, an 18-year-old shopkeeper in the market told AFP from his hospital bed.

Khan sustained injuries in his right shoulder and legs.

Anwar Ali, a passenger in the bus, said the blast overturned the vehicle.

"I was sitting in the front seat when a powerful wave struck me and my head hit the front windscreen. I don't know what happened after that," Ali told AFP.

In an earlier incident, militants blew up the election office of Kamran Khan, a former legislator from North Waziristan who supported the outgoing government led by the Pakistan People's Party. No one was hurt.

On Thursday, militants shot dead a candidate for outgoing coalition partner the Muttahida Qaumi Movement (MQM), the first to die in the election campaign, in an attack claimed by the Taliban.

Pakistan says more than 35,000 people have been killed as a result of terrorism in the country since the 9/11 attacks on the United States.