Troops moved on Miramshah, the main town in North Waziristan, around 9:30am, officials said, after two weeks of air strikes and artillery bombardment of insurgent targets.
Washington has long urged Islamabad to tackle militant sanctuaries in North Waziristan, a haven for Tehreek-e-Taliban Pakistan (TTP) and al Qaeda-linked insurgents.
Security officials in Miramshah told AFP troops moved on the town after six hours of artillery shelling, which began at 3:30am.
"They are using tanks and armoured vehicles for ground movement," a security official said on condition of anonymity.
"The forces have also taken over the civil hospital in Miramshah and have destroyed four hideouts in the artillery firing."
Other security officials in Peshawar confirmed the advance.
A statement from the military's communications wing said the assault began "after the evacuation of all civil population", and infantry troops were carrying out house-to-house searches.
Troops have killed 15 militants so far in the ground attack, the military said, while three soldiers were wounded in clashes.
Since the operation began on June 15, 376 militants have been killed and 19 have surrendered, according to the military, while 17 soldiers have also died.
The conflict zone is off-limits to journalists at the moment, making it impossible to verify the number and identity of those killed.
On Saturday the army said a leading TTP commander was killed, while a "prominent" al Qaeda commander was arrested while trying to flee.
Nearly 500,000 people have fled North Waziristan because of the operation, codenamed "Zarb-e-Azb" after a sword used in battle by Prophet Mohammad (pbuh).
Tens of thousands of families have left for the town of Bannu, close to North Waziristan, while hundreds more have moved further afield to the towns of Lakki Marwat, Karak and Dera Ismail Khan since the offensive began in mid-June.
The exodus has created a huge humanitarian challenge for Pakistan, and there have been complaints and protests at the slow delivery of aid in Bannu.
Monday was the first day of the Muslim fasting month of Ramadan in Pakistan, and many of those displaced by the fighting face spending the month in difficult conditions in camps.
Tens of thousands have also fled across the porous mountain border into Afghanistan, and there have been fears that key militants may have slipped out.
Prime Minister Nawaz Sharif has said the operation will mark the "beginning of an era of peace and tranquility", but the Taliban have vowed to strike back.
Nawaz, elected last year vowing to bring an end to the Taliban's insurgency through dialogue, had doggedly pursued talks with the TTP since February to little avail.
A dramatic attack on Karachi airport which killed dozens of people brought a sudden, bloody end to the peace process and sparked the military offensive.
Pakistan's allies, particularly the United States, have long called for an operation in the mountainous tribal territory to flush out groups like the Haqqani network, which use the area to target Nato troops in neighbouring Afghanistan.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ