At least nine people have been killed and seven others wounded after a bomb exploded on a bus in Pakistan's northwestern city of Peshawar, officials say.

Fazal Wahid Khan, a senior police officer, confirmed the toll to the AFP news agency, saying bomb disposal officials told him that Saturday's blast was a "timed device".

Shafi Ullah Khan, another police official, confirmed the attack in the city's Matani suburb for which no one has claimed responsibility so far.

The dead and injured included women and children, and authorities transported them to a government hospital, an official said.

Abdul Haq, a senior bomb disposal expert told AFP that four to five kilogrammes of highly explosive material was used.

Bus driver Jehanghir Khan, who was not hurt, told reporters that the device had been planted under a seat.

"I didn't see any suspicious person. I don't know who planted this bomb. I was passing through the bazar of Matni when I suddenly heard the blast," he said.

Fresh military push

He said the blast damaged some shops nearby, as well as the bus.

"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.

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

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 fighters.

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

Peshawar, capital of troubled Khyber Pakhtunkhwa province which borders Afghanistan, is one of Pakistan's most volatile regions and suffers deadly attacks almost on a monthly basis, many blamed on the Pakistani Taliban.

Earlier, this month dozens of gunmen attacked an electricity plant in the same region, killing seven people and disrupting power to 100,000 people overnight, officials say.