A curfew has been enforced across the district of Bannu
The Pakistani army has begun shelling Taliban hide-outs in the Bannu district of the country's north-west, officials and local people say.
Bannu's district coordination officer, Kamran Zeb Khan, told the Associated Press news agency that the shelling began on Tuesday morning.
He said the order was given after a deadline expired for tribal leaders to hand over militant suspects.
Correspondents say it is unclear if the shelling means a new front is opening.
"I can hear the artillery fire in the area as forces have started pounding militant hideouts," local administration official Muwaz Khan told the AFP news agency, adding that "hundreds" of troops have arrived in the area from two nearby towns.
Police in Bannu say they fear militants may head towards the area
The army is already carrying out an air and ground assault against the Taliban in three districts in and around the Swat valley.
Bannu sits alongside South and North Waziristan, two semi-autonomous tribal districts where al-Qaeda and the Taliban are believed to be entrenched.
Waziristan has been described by US officials as "the most dangerous place on earth".
It is said to harbour some of the world's most wanted men including al-Qaeda leader Osama Bin Laden and his deputy, Ayman al-Zawahiri.
A curfew was imposed in Bannu earlier on Tuesday. Police told the BBC that they had imposed the curfew in six of the 12 police station districts.
"The six police stations all border the semi-tribal areas of Janikhel and Bakakhel, as well as the North Waziristan tribal region," Mohammad Iqbal, head of police in Bannu said.
"We fear that with the impending military operation in Janikhel, the militants may escape towards Bannu."
Mr Iqbal added that all bus stops in Bannu had been closed in an attempt to thwart militant attacks.
Local authorities there have also sealed businesses and properties belonging to members of the Janikhel and Bakakhel tribes.
The BBC's Syed Shoaib Hasan in Islamabad said that the Janikhel tribal area has been long known as a hide-out for militants.
Since 2008 it has been the target of several missile strikes in which dozens of Taliban and al-Qaeda members have reportedly been killed.
The authorities say the tribes in the area have aided Taliban militants in their fight against security forces - who have been steadily increasing their numbers in Janikhel.
"Hundreds of additional troops have arrived here," Abdul Razzaq, a local administration official in the Janikhel area, told the BBC.
"The entire region has been sealed off from all sides.
"Most of the locals have already left the area and gone to their relatives in Lakki Marwat and Bannu," he said.
Some locals in the area told the BBC that the army operation in Janikhel would have no effect on the militants because they fled the area two days ago when the authorities announced a general evacuation.
The army's move comes days after dozens of college students and teachers on their way to Bannu were kidnapped in North Waziristan.
The mass kidnapping took place in Janikhel and is said to have been carried out by local Taliban militants loyal to Tehrik-e-Taliban (TTP) head Baitullah Mehsud. The kidnapped students have all been released.
The specific target of this operation is said to be Mehsud and his TTP organization, the BBC's Syed Shoaib Khan says.
Our correspondent says that the plan appears to be to encircle Mehsud and destroy his organisation.
Further north the Swat valley has been the scene of heavy fighting between the Pakistani military and Taliban militants as the army attempts to dislodge militants from their strongholds in that area.
Helicopter gunships in the Upper Dir region on Tuesday were deployed to support hundreds of tribesmen fighting the Taliban, officials say.
The tribesman blame the Taliban for a bomb attack on a mosque that killed 35 people on Friday.
Officials say they have been joined in their fight by residents from two villages and a town.
There are now about 2,000 of them fighting 200 surrounded Taliban militants, officials say.