The explosion targeted passengers in a motorized rickshaw and those on a mini-bus in Kohat, some 150 kilometers (100 miles) west of the capital, Islamabad, police official Iqbal Khan said.
No one immediately claimed responsibility for the attack, though Kohat has seen past attacks by local Taliban fighters and allied sectarian groups against its minority Shiite population, which has a presence in the city and its outskirts.
Khan and police official Fazal Naeem said the vehicles targeted were bound for a Shiite-majority area, and suspected that the minority Muslims could have been the target of the attack.
Northwestern provincial police chief Nasir Durrani said security forces have been conducting counterinsurgency operations in the area.
Local Taliban militants have killed tens of thousands of Pakistanis in a bloody war against the state in a bid to overthrow the government and enforce their own harsh brand of Islamic Shariah law.
Pakistan's government recently started peace talks with the Taliban, but negotiations were suspended after the killing of 23 soldiers by a faction of the militant group and a militant-claimed bombing in southern port city of Karachi that killed 13 police officers.
Air force jets have been pounding militants' hideouts in various tribal regions near the Afghan border since the peace talks collapsed last week.
Both the Pakistani government and the local Taliban are demanding each other to initiate a cease-fire first to resume the talks.
Pakistani Prime Minister Nawaz Sharif long has favored peace talks over military action to end the bloodshed in the northwest, but he is also under pressure from critics to retaliate for any Taliban violence.
One of the critics is an ethnic political party based in Karachi called the Muttahida Qaumi Movement.
The party on Sunday held an anti-Taliban rally to express solidarity with Pakistan's security forces. Thousands of people demonstrated and were addressed by the party leader Altaf Hussain by phone from London.
"Taliban are cancer for Pakistan. Taliban are cancer for the humanity," he said.
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