A "full-fledged operation" had been launched in the central part of Kurram Agency, a senior unnamed official in the northwestern city of Peshawar was quoted as saying by Dawn newspaper.
The government had notified 80 sq km in the region as a conflict zone a fortnight ago.
"I have no specific details but the operation has started in the mountainous area," the official said.
Troops were dropped by helicopters in Manato and Zaimukhet areas.
Army and paramilitary forces, backed by the air force, were conducting a joint operation in the areas.
Thousands of troops were taking part in the operation, the Dawn quoted its sources as saying.
However, The News daily quoted the chief military spokesman Maj Gen Athar Abbas as saying the operation had not yet been launched but preparations were underway for taking action against militants in Kurram Agency.
He contended that militants would flee their hideouts if reports about a likely military operation appeared in the media before action was actually started.
But, the daily said the Pakistan Army contingents backed by gunship helicopters, tanks and artillery had moved into troubled spots of Kurram Agency to take on militants affiliated with the Tehrik-e-Taliban Pakistan led by Hakimullah Mehsud.
Troops had entered militant strongholds in the mountainous central part of Kurram Agency, the sources said.
Local residents said they heard artillery and tanks pounding suspected militant positions in Zaimusht, Munda, Khoaidadkhel and Alisherzai areas.
There was no information about casualties suffered by the militants.
The security forces had occupied hilltops while militants were still present in remote villages in the mountains and were allegedly forcing residents not to leave their homes.
Curfew was imposed and communications across Kurram Agency were suspended before the launch of the operation.
Some 4,000 families had fled their homes for safer areas, The News reported.
The displaced families were moving to a relief camp set up in Durrani area, reports said.
The Disaster Management Authority in the tribal belt had set up two registration centres where 500 displaced families had enrolled themselves so far, the reports said.
The central part of Kurram agency is adjacent to Tora Bora, reportedly the stronghold of al-Qaeda in Afghanistan.
Militants pushed out of Waziristan and other parts of Pakistan`s tribal areas and Khyber-Pakhtunkhwa province have set up sanctuaries in central Kurram.
Hundreds of people have died in bloody clashes between rival Shia and Sunni tribesmen in Kurram Agency over the past three years.
Taliban fighters who infiltrated the region have been backing the Sunnis.
Most parts of the region are cut off from the rest of Pakistan as the militants have blocked key roads.
However, it was not clear whether the Pakistani troops would fight Fazal Saeed Haqqani, who recently parted ways with the Tehrik-e-Taliban Pakistan and formed his own group called Tehrik-e-Taliban Islami.
Haqqani said he had deserted the Taliban because it was carrying out suicide attacks and targeting civilians.
Haqqani claimed he was affiliated with the powerful Haqqani network that fights US-led forces in Afghanistan.
Members of the Haqqani network have disowned him.