The army said it retaliated, causing heavy casualties, but independent confirmation was not immediately possible because the districts along the mountainous Afghan border are off limits to journalists and human rights organisations.
"Militants from inside Afghanistan across the international border opened fire on Pakistani troops in Kurram district," the military's media wing said in a statement.
The Tehrik-e-Taliban Pakistan (TTP) or Pakistan Taliban claimed responsibility for Sunday's attack.
The Afghan government denied the firing had come from within Afghan territory.
"We assure other countries, especially our neighbours, that no one will be allowed to use Afghan land against them," Bilal Karimi, deputy spokesman for Afghanistan's Taliban government, told Reuters.
The Afghan Taliban late last year played the role of facilitator in talks between the TTP and the Pakistan government.
Those talks fell apart in December, since when there have been a series of attacks on Pakistani forces along the border.
The military statement said Pakistan strongly condemns the use of Afghan soil by militants for activities against Pakistan and expects that interim Afghan government to halt such acts in future.
Pakistani security forces had just a day earlier completed a three-day operation against militants who had attacked two military bases in the southern province of Balochistan.Â
At least nine soldiers were killed in those attacks.
"As per its promises, Taliban government should stop such cross-border militants attacks," Sheikh Rasheed Ahmad, Pakistan's interior minister, said in a statement.
