A suicide bomber and several gunmen detonated an explosives-laden vehicle near the post in Bannu, a district in Khyber Pakhtunkhwa province bordering Afghanistan, late Saturday, said senior police official Sajjad Khan.
The attack triggered an intense shootout, and some officers were killed in the exchange, while others died later after the building collapsed.
Rescuers conducted a search operation using heavy machinery to retrieve bodies from under the rubble, Khan said, adding that three police officers were wounded in the attack.
Security forces have also launched an operation to track down the perpetrators.
A newly formed militant group, Ittehad-ul-Mujahideen Pakistan, claimed responsibility in a statement sent to reporters.
While the group claims it was formed by splinter factions of the Pakistani Taliban, known as Tehrik-e-Taliban Pakistan, authorities have accused it of being a front for the TTP.
Images after the attack on Saturday showed the structure had been reduced to rubble, with bricks, charred wreckage, and mangled vehicles scattered around the area.
A police official who asked not to be identified said militants first rammed into the post with an explosive-filled car and then entered the premises and began firing on any remaining officers.
"Other law enforcement personnel were sent to help the police, but the terrorists ambushed them and caused some casualties," he said.
Police sources said the militants also used drones in the attack.
Ambulances from rescue agencies and civil hospitals rushed to the scene, with officials saying a state of emergency had been declared in government hospitals in Bannu.
Militant attacks have the potential to reignite fighting along Pakistan's border with Afghanistan.
The worst fighting in years erupted between the allies-turned-foes in February, with Pakistani airstrikes inside Afghanistan that Islamabad said targeted militant strongholds.
Fighting has since eased, with occasional skirmishes breaking out along the border, but no official ceasefire has been brokered.
Islamabad blames Kabul for harbouring militants who use Afghan soil to plot attacks in Pakistan.
The Taliban has denied the allegations and said militancy in Pakistan is an internal problem.
