Security in Pakistan has deteriorated significantly since mid-2007, with tribal fighters linked to the Taliban and al-Qaeda carrying out bombings and suicide attacks.
More than 600 people have been killed in the violence since the beginning of this year.
Sunday's attack came amid efforts by the new government - sworn in at the end of March - to broker a peace deal with tribes in the semi-autonomous areas bordering Afghanistan, where the violence has been most pronounced.
Wajid Ali Khan, a government official in the North West Frontier Province, condemned the attack as "cowardly."
He said: "We are engaged in efforts for peace and stability in this province and this seemed to be an act to sabotage our efforts."