Pakistani fighter jets resumed air strikes in North Waziristan on Monday, a day after the army announced the start of a major military operation to flush insurgents out of the volatile region bordering Afghanistan.
Military sources said combat aircraft pounded targets in the region's Shawal sector, killing at least 21 militants.
Independent confirmation or other details were not immediately available in a region where journalists are not allowed to operate freely.
The army has imposed an all-day curfew and turned off mobile phone services to undermine the insurgency and restrict people's movements, leading to food shortages in some places.
Expecting an escalation of violence, two-thirds of families have fled from the ethnic Pashtun region, residents said, many heading for neighbouring Afghanistan, where they have relatives.
"We have packed up everything and are ready to leave as soon as the curfew is lifted," said Ethasham Khan, a resident of the regional capital of Miranshah, its usually bustling streets empty of people and traffic.
The curfew will be relaxed in the next couple of days to allow residents to leave the area, a security official said.
On Sunday, Pakistan sent troops, artillery and helicopter gunships to North Waziristan in a long-expected military operation seen as a response to a deadly insurgent attack on its biggest airport a week ago.
For now, ground troops - now numbering some 80,000 in North Waziristan, according to military sources - have not been involved in direct military action, leaving F-16 combat jets to conduct air strikes.
It was also unclear how long officials expect the operation to last in a region of forbidding mountainous terrain that has historically never been subdued by any government.
In a telling detail, the operation is called Zarb-e-Azb in Urdu, or "Strike of the Prophet's Sword".
REUTERS