U.S. and Afghan forces struck al-Qaida militants in Afghanistan's Tora Bora region as part of an ongoing offensive Thursday.
Officials say airstrikes and hundreds of ground forces are targeting insurgent support bases and training sites in Nangarhar province near the Afghan-Pakistan border.
Casualty figures have not been confirmed, but Afghan media report 50 militants have been killed.
Afghan officials say at least 100 families have fled the area.
U.S. officials have not said whether senior Taleban or al-Qaida leaders are targets of the operation. The insurgent leadership is believed to be hiding in the Afghanistan-Pakistan border region.
A Pakistani military spokesman, General Waheed Arshad, says Pakistani troops have reinforced the border near the Kurram tribal region in order to stop militants from crossing into Pakistan from the Tora Bora region.
Local Pakistani residents have been reporting seeing NATO warplanes flying overhead in recent days.
U.S. military officials say the current operation is aimed at pinpointing enemy positions and avoiding civilian casualties.
Coalition and NATO forces have been criticized for accidentally killing Afghan civilians.
The remote and mountainous terrain of Tora Bora was heavily bombarded in late 2001 by U.S. forces hunting for al-Qaida leader Osama bin Laden. He is believed to have escaped the assault.
Some information for this report was provided by AFP.