Police spokesman Fazal Naeem says 15 police were also wounded in the attack Thursday in Hangu district in Khyber Pakhtunkhwa province. The post was manned by both normal and paramilitary police.
No group has claimed responsibility for the attack.
Suspicion will likely fall on the Pakistani Taliban, who have carried out many similar bombs against security forces in the northwest.
A roadside bomb hit a vehicle carrying members of an anti-Taliban militia in northwestern Pakistan on Thursday, killing seven militiamen, a police spokesman said.
Nine members of the militia were also wounded in the attack in Stanzai village in the Orakzai tribal region bordering Afghanistan, said Fazal Naeem. The militiamen were on their way to a meeting to discuss strategy against the Pakistani Taliban at the time of the attack, he said.
No one claimed responsibility for the blast but suspicion fell of Pakistani Taliban who have carried out many such attacks and killed scores of militiamen for supporting military operations against the Taliban.
The latest attack came hours after five suicide bombers attacked a police station in the country's northwestern city of Bannu, wounding one police officer.
The city's police chief Nisar Tanoli said three of the bombers detonated their explosives vests while the police shot dead the other two.
Pakistani Taliban spokesman Ahsanullah Ahsan claimed responsibility for the attack in Bannu, describing it as retaliation for the killings of eight militants whose bullet-riddled bodies were found abandoned in the neighboring North Waziristan tribal area.
Thursday's attacks - both in the country's northwest but in different areas - came at a time when many of the country's main political parties were meeting in the capital, Islamabad to discuss an offer of peace talks by the Pakistani Taliban.
Pakistan's interior minister Rehman Malik has said the government was ready to hold peace talks with the Taliban, who have been waging a bloody insurgency in Pakistan that has killed thousands of people.
Associated Press Writers Ijaz Mohammad from Bannu and Riaz Khan from Peshawar contributed to this report.