Seven Pakistani soldiers were beheaded by fighters who crossed over from Afghanistan, the Pakistani army has said, amid ongoing gun battles in the border region.
Several other soldiers were also missing, the army said on Monday, and none of the bodies had been recovered yet.
Pakistan claimed that more than 100 fighters entered Upper Dir, a district in northwest Pakistan, after crossing from neighbouring Kunar province in Afghanistan.
The military had already reported six soldiers killed in gun battles with fighters on Sunday.
The Pakistani government lodged a protest over the raid with NATO and Afghan forces, accusing them of failing to act against fighters' safe havens in Afghanistan.
The NATO-led International Security Assistance Force (ISAF) in Afghanistan said it was aware of the report, but had no further information.
General John Allen, the ISAF commander, is due to visit Pakistan later this week, a trip that could focus on improving tense relations between the two sides.
'Safe havens' on both sides
Intelligence officials blamed the attack on followers of Maulvi Fazlullah, a cleric who fled to Afghanistan after the Pakistani army forced him out of the Swat Valley in a 2009 offensive.
He re-emerged as a threat last year, when his fighters conducted cross-border raids that killed around 100 Pakistani security forces.
Fazlullah Wahidi, the governor of Kunar province, said the assailants were based in Pakistan, not Afghanistan.
"We don't have any information about militants crossing the border from Afghnaistan to attack troops in Pakistan," he told the Reuters news agency.
Pakistan wants Afghan and NATO forces to act against armed groups based in Afghanistan. The US has called on Pakistan to get its own house in order and target Pakistan-based fighters who cross the border in the other direction.
Leon Panetta, the US defence secretary, said earlier this month that Washington was reaching the end of it patience over the matter.