Pakistani military aircraft have bombed suspected Taliban positions in North Waziristan, following a wave of attacks against security forces.
A military official said 40 militants had been killed in the air strikes in the tribal area bordering Afghanistan. The figure cannot be confirmed.
On Sunday, at least 20 soldiers were killed in an ambush on an army convoy.
Pakistan's Taliban vowed to step up attacks after a drone strike last year killed their leader Hakimullah Mehsud.
The air strikes started at around 00:30 local time (19:30 GMT on Monday), an official told AFP news agency, adding that helicopter gunships also took part.
The air assault comes after the military attacked militant targets in North Waziristan with artillery fire and helicopter gunships in December.
Pressure for action
Two residents interviewed by the Associated Press news agency over the telephone said they slept in the open, fearing their homes would be hit.
The army said 40 "terrorists" had been killed and claimed they had all been involved in recent militant attacks - although there is no independent confirmation of this.
Tribal elders and the Taliban put the death toll lower. Some reports said there were civilian casualties, but this is also impossible to verify as there is no media access to the area.
Past army operations have killed civilians, the BBC's Aleem Maqbool in Islamabad says, and it is feared this is the case this time too.
But the authorities in Pakistan have been under pressure to take decisive action against the Pakistani Taliban.
The government's repeated offers of talks had come to nothing, and the militants had not refrained from attacks, our correspondent says.
In the past month, the Pakistani Taliban have launched a string of deadly attacks after weeks of relative calm since Hakimullah Mehsud was killed.
On Sunday, soldiers and paramilitary personnel were preparing to leave Bannu, Khyber Pakhtunkhwa province, for Razmak in North Waziristan when their convoy was hit by a bomb.
On Monday, a suicide bomber killed at least 13 people near Pakistan's army headquarters in Rawalpindi, south of the capital Islamabad. Among those who died were eight military personnel.
The Pakistani Taliban claimed both attacks.
Their new leader, Mullah Fazlullah, has vowed to continue attacks against Pakistani forces.
Prime Minister Nawaz Sharif - under pressure to take a tougher stand on militants - cancelled his trip to the World Economic Forum in Davos after Sunday's attack.
The two attacks were among the most serious assaults on the military in recent months and have called into question Pakistan's strategy for dealing with militancy.