Pakistani troops backed by jet fighters and artillery have killed about 50 militants in a volatile northwestern tribal region near Afghanistan where the country's top Taliban leader is believed to be entrenched with thousands of his fighters, officials said Saturday.

They were the first known militant casualties in South Waziristan — where Pakistan Taliban head Baitullah Mehsud and Al Qaeda figures are believed to be hiding — since the military started pounding the area with artillery about a week ago. Mehsud is blamed for a series of suicide attacks that have killed more than 100 people since late May.

Although the army has not announced a formal start of full-scale operations in South Waziristan — an offensive that Washington has been pressing Pakistan to undertake — officials said troops are already occupying strategic positions in the region.

The operation, seen as a test of nuclear-armed Pakistan's resolve against an insurgency that has expanded in the past two years, could be a turning point in its sometimes halfhearted fight against militancy. It also could help the war effort in Afghanistan, because the tribal belt has long harbored militants who launch cross-border attacks.

Jet fighters flattened two abandoned militant-linked seminaries and a training facility Friday in a clear sign that the operation was ramping up.

Two intelligence and army officials, speaking on condition of anonymity because they were not authorized to speak to media, said heavy fighting was under way in the villages of Barwand and Madijan, with about 50 militants killed.

There was no immediate comment from the military, and the reports could not be independently confirmed due to restrictions on media access to the region.

Meanwhile, artillery fire was pounding militant positions in the Biha valley, in the upper Swat Valley, as an intense operation there started Friday night against remnants of Taliban fighting forces and continued into the day.

"This area is the center of gravity for the terrorists," said Maj. Gen. Sajjad Ghani, who is in control of efforts to clear a 3,860-square-mile area of Taliban. "As of now, there are only pockets of resistance left. The terrorists are on the run. Command and control is disarray. They are unable to organize an integrated response."

During a military-sponsored trip for journalists to the town of Chuprial, Ghani said 95 percent of Swat has been cleared and that most of the resistance the military is facing is in Biha, a short valley that backs into snow-covered mountains that are limiting the Taliban's efforts to flee.

He said about 400 militants have been killed in the area over the past six weeks but conceded that many top commanders have managed to escape, some possibly headed to havens in Afghanistan or the Waziristan tribal areas. He did not specify how many militants are believed to be holding out, and his statements could not be independently confirmed.

Overall, the army says it has killed nearly 1,500 militants since April in Swat.

Ghani said a high-intensity operation will continue for about a week or so, then another few weeks will be needed to go after stragglers.

Reporters were taken to an abandoned militant training camp where Ghani said about 50 militants were killed, including Arabs, Afghans and Uzbeks. The complex included tunnels and an ammunition dump. Troops showed off seized weapons, including improvised bombs, heavy machine guns and ammunition boxes for rocket-propelled grenades.

Helicopters, including Cobra gunships, flew overhead, and there was no sign of civilians in the scenic area of steep mountainsides and terraced fields, dotted with small villages of single-story concrete houses. The army clearly has the high ground in most places, dug in with heavy machine guns in sandbagged bunkers.

Officials are planning to let some of the 2 million people displaced by fighting in Swat to start returning home further south Thursday.

They are being sent first to Mingora, Swat's main city. Electricity and civic facilities must be restored before they are allowed to go home in "phases," said Fazal Karim Khattak, a senior government official.

Refugees were happy to hear they will soon go home but worry about what they will find.

"Of course I am happy, but I don't know whether our home is safe or it has been destroyed," said Khadija Bibi, 45, a mother of four who left her home in the Kanjua near Mingora in May.

Khaisata Khan, 32, who owned a shop in the heart of the city of Swat, said he didn't know what had happened to his shop as the military had targeted Taliban in the area where it was located.

"If peace returns to Swat, I will forget the damage to my property and the pain we have to face in the camps," he said as he sat in a camp on the outskirts of the main northwestern city of Peshawar.

The Swat offensive has been generally welcomed in Pakistan, but public opinion could quickly turn if the government fails to effectively help the refugees or civilian casualties mount. The government has said the army will need to stay in Swat for a year to ensure security.