The Pakistani army has begun setting up checkpoints in the areas of south Waziristan it has taken over from the Taliban.
Early success is being claimed in the push into the tribal territory, with heavy fighting reported on a front near Shakai, and streams of people escaping the combat.
They are being filtered for suspected Taliban members before being allowed to take refuge in the rear.
The army says the bulk of the operation is attempting to close a pocket in the northern part of south Waziristan in which it believes the main body of Mehsud fighters is based. There may be Uzbek and al-Qaeda units fighting alongside them.
The US’s central command chief for Iraq and Afghanistan General David Petraeus has backed the offensive and is meeting a range of Pakistani officials during a visit.
He is being kept informed of the offensive’s progress, but reporting from the combat zone is unverifiable and there are claims and counter claims about casualties and territorial gains by both sides.