Pakistani forces have pounded Taliban positions with heavy artillery, carrying a major operation in South Waziristan into its third day and reportedly killing up to 20 militants.
The rugged district, part of a tribal belt on the Afghan border, is home to thousands of fighters linked to the Taliban and al Qaeda.
Troops trained heavy artillery on Taliban bases after advancing on three fronts, pressing an offensive that was in the planning for months.
Between 17 and 20 militants have been killed so far in the operation, according to security officials, but the figures are impossible to check: all non-military access to the region has been cut off.
A sixth of the population has fled
Some 20,000 people have fled from the conflict zone to the nearby town of Dera Ismail Khan, according to a BBC correspondent. Since August, at least 90,000 people have fled South Wazirist - normally home to 600,000.
Pakistan has mounted numerous offensives against militants in its northwest since 2002, meeting with limited success and paying for it with the lives of 2,000 troops.
This time, commanders have outlined an offensive lasting six to eight weeks, with the goal of finishing before the onset of harsh winter snows.
There are an estimated 10,000 to 12,000 fighters from the Tehreek-e-Taliban movement in South Waziristan and up to 25,000 across Pakistan's semi-autonomous tribal belt, which has a history of fierce independence.