Two bomb attacks Sunday targeting the offices of election candidates in northwest Pakistan killed at least eight people, officials said, the latest bloodshed ahead of polls next month.

Violence has marred the campaign for the landmark May 11 general election, with more than 50 people dead in blasts and suicide attacks since April 11, according to an AFP tally, including more than 20 in the past three days.

Five people died and 22 were wounded on Sunday in a blast at the office of an independent candidate in the garrison city of Kohat, adjacent to Pakistan's restive tribal areas along the Afghan border.

A blast at the offices of another independent candidate in Peshawar, the main city of the northwest, killed three people and wounded 13, officials said.

The Kohat blast came at the campaign office of Noor Akbar Khan, who is running in the Orakzai tribal district. He is canvassing for votes among the numerous Orakzai residents forced to flee the district to Kohat by army operations against militants.

"The IED was planted outside the election office. At least five people have been killed and 22 injured," city police chief Dilawar Khan Bangash told AFP.

Fazal Naeem, a police spokesman in Kohat, confirmed the attack and told AFP the blast had damaged shops and vehicles nearby and also hit an office of the Awami National Party (ANP), which has been targeted repeatedly by the Taliban.

"The election office was open at the time and supporters of Noor Akbar Khan were sitting inside. The death toll may rise, the condition of some of the injured is critical," Tanveer Khan, another police official, told AFP.

Nasir Khan Afridi, the independent candidate whose office was hit in Peshawar, is running for a seat in Khyber tribal district and like Khan, has been campaigning among voters displaced by a military offensive.

On Saturday three blasts in Karachi, Pakistan's biggest city and commercial heart, killed at least three people and wounded 49.

Two of the bombs hit the offices of the Muttahida Qaumi Movement (MQM) and the Pakistan People's Party (PPP), which along with the ANP were coalition partners in the outgoing national government, led by the PPP.

The Pakistani Taliban has targeted the three parties, which are perceived as secular and backed military operations against the Islamists.

As a result of the threats, there have been few large-scale political rallies leading to a lacklustre campaign for the elections.

Amnesty International has also called on Pakistan to investigate the recent wave of attacks and ensure adequate protection for election candidates.

May 11 national polls should see power pass from a civilian government that has served a full term to another through the ballot box for the first time in the nuclear-armed country's turbulent history.