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.
"These people have a very big role still," he said. Maj Gen Zaheer ul-Islam Abbasi, a former intelligence officer who was convicted of attempting a coup against the government of Benazir Bhutto in 1995 and who is now dead, was one of the most active supporters of the militant groups in the years after Sept 11, the former commander said.
He said he saw General Abbasi several times: once at a meeting of Taliban and Pakistani militant leaders in Khyber-Pakhtunkhwa Province as they planned how to confront the American military in Afghanistan; and twice in Mir Ali, which became the centre for foreign militants in Pakistan's tribal areas, including members of Al Qaeda.
There were about 60 people at the Taliban meeting in late 2001, soon after the Taliban government fell, the former commander said.
Pakistani militant leaders were present, as were the Taliban ambassador to Pakistan, Abdul Salam Zaeef, and Muhammad Haqqani, a member of the Haqqani network.
Several retired officials of Pakistan's premier spy agency, the Directorate for Inter-Services Intelligence, or ISI, were also there, he said, including a man known as Colonel Imam but who was actually Brig Sultan Amir, a well-known trainer and mentor of militants, and General Abbasi.
The militant groups divided Afghanistan into separate areas of operations and discussed how to "trip up America," he said.
The Pakistani military still supports the Afghan Taliban in their fight to force out American and NATO forces from Afghanistan, he said, adding that he thought they would be successful.