In response, Pakistan has closed the border crossing for supplies bound for Nato forces in Afghanistan.
Nato said it was aware of "an incident" near the border and was investigating.
The alleged attack took place at the Salala checkpoint, about 1.5 miles (2.5 km) from the Afghan border, Reuters reports, at around 02:00 local time (21:00 GMT).
If confirmed, the attack would further complicate US-Pakistan relations, already under strain following a unilateral US raid that killed Osama Bin Laden in Pakistan in May.
At least seven soldiers were wounded.
The military gave no official toll, saying casualties were reported but details were awaited.
'Supplies halted' Masood Kausar, governor of Khyber Pakhtunkhwa province, condemned the attack in a statement, reported AFP news agency.
"Such cross-border attacks are unacceptable and intolerable," he said, adding that the government would take up the matter at the highest level and launch a thorough investigation.
In apparent response to the attack, lorries and fuel tankers were being stopped at Jamrud town in the Khyber tribal region near the city of Peshawar, officials and local media said.
"We have halted the supplies and some 40 tankers and trucks have been returned from the check post in Jamrud," Mutahir Zeb, a senior government official, told Reuters news agency.
Pakistani troops are involved in fighting the Taliban in the crucial border region area. Hundreds of militants have been resisting attempts by the security forces to clear them from southern and south-eastern parts of the district.
The checkpoint at the centre of this latest incident was set up to prevent insurgents crossing over the border into Afghanistan, reports the BBC's Shoaib Hasan in Islamabad.
He says the movement of insurgents from the area into Afghanistan has been a concern for the Nato-led International Security Assistance Force and the US.
The US has been targeting militants in Pakistan's tribal areas near the Afghan border for several months, often using unmanned drone aircraft.
Last year, US helicopters accidentally killed two Pakistani soldiers near the border, also prompting Pakistan to temporarily close the border to Nato supplies.
In October, Pakistan's army chief Ashfaq Kayani warned the US against taking unilateral action in nearby North Waziristan.
He said that the US should focus on stabilising Afghanistan instead of pushing Pakistan to attack militant groups in the crucial border region.
Washington has for many years urged Islamabad to deal with militants in the area.
