According to a statement issued by the Pakistani army, operations had begun around Miranshah, the main town in the tribal region bordering Afghanistan.
The move follows air strikes which the army says have killed 370 militants. There is no confirmation of the figure.
North Waziristan has long been a sanctuary for militants. Correspondents say many are thought to have left the area before the offensive began.
The assault comes three weeks after militants attacked Pakistan’s largest airport in Karachi, leaving more than 30 people dead.
For the past two weeks, Pakistani forces have been carrying out air strikes against what it says are militant hideouts in North Waziristan.
Among their targets, they say, have been Uzbek militants who claimed responsibility for the Karachi attack and their Pakistani Taliban (TTP) allies.
Monday’s army statement said troops were now conducting a door-to-door search in Miranshah.
“Troops have recovered underground tunnels and IED [improvised explosive device] preparation factories,” the statement said.
The town has been one of the main TTP bases during recent years when militants who had at times been tolerated by the military killed thousands of people in a bombing campaign across Pakistan.
In public statements, Pakistani commanders have said they will not discriminate between so-called good and bad Taliban.
However, there are widespread reports from within North Waziristan that many militants were allowed to escape before the operations began.
Nearly half a million people have left North Waziristan since the offensive was announced following the Karachi airport attack.
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