Pakistan is bracing itself for huge street protests amid a political crisis.
Move follows air strikes that the army says have killed 370 militants.
About 170 militants have been killed in Pakistan's northwest.
Army hopes new offensive will disrupt militant strongholds.
Attack comes days after militants attacked a Karachi airport.
Local media say gunmen attacked a camp of the airport security forces.
Taliban militants disguised as security forces stormed Pakistan's busiest airport on Sunday and at least 27 people were killed in a night-long battle at one of the country's most high-profile targets.
Karachi entered its second day of lockdown as tensions grow over the London arrest of a powerful party chief.
Police arrested Altaf Hussain on charges of money laundering.
Deny they stood by while pregnant woman was stoned by relatives.
