Pakistani forces killed 80 militants and drove the Taliban from a major urban stronghold, the army said, as U.S. military planes brought aid for civilians fleeing fierce fighting in the northwest.

In an indication that the fighting in the Swat Valley area and the resulting humanitarian emergency may drag on, a U.S. military official predicted that 250,000 refugees will still be in camps at the end of the year.

Pakistani troops launched an offensive last month after Taliban militants based in Swat pushed into the district of Buner, bringing them within 60 miles of the capital of Islamabad and prompting intense U.S. pressure for a stiff response.

Government forces cleared Sultanwas, the main Taliban-held town in Buner, overnight following intense clashes, army spokesman Maj. Gen. Athar Abbas said.

He said troops destroyed several vehicles used by black-clad militants and defused a string of homemade bombs.

"Sultanwas was the main stronghold of terrorist-miscreants in Buner, where they have made concrete underground bunkers and ammunition dumps," Abbas said at a news conference.

The army claims it has killed more than 1,000 militants and won back swaths of territory from militants in Swat, a valley whose scenery and cooler climate once drew hordes of summer tourists.

More than 50 troops have died, including one soldier Wednesday as troops battled insurgents entrenched in several other key towns in the valley, Abbas said.

However, authorities say the clashes have prompted about 1.9 million people to flee their homes, creating a humanitarian crisis that could sap Pakistani enthusiasm for similar action against Taliban and Al Qaeda sanctuaries near the Afghan border.

Pakistani generals have refused to predict how long it will take to eliminate militants from Swat. Two previous offensives ended in short-lived peace deals from which the militants emerged stronger.

Still, the top U.S. military official at the U.S. Embassy in Islamabad forecast Wednesday that between 200,000 and 250,000 will be living in refugee camps at least until the end of 2009.

Rear Adm. Michael A. LeFever also said the Pakistani military had set up roadblocks to keep militants away from the camps, just south of the battle zone.

Relatives have taken in most of those driven out of Swat in fear of their lives. But about 160,000 refugees have registered so far at the camps, Information Minister Qamar Zaman Kaira said Wednesday. Many thousands more are believed to be hunkered down in their homes in Swat, unwilling or unable to move.

Abbas, the Pakistani army spokesman, said the military had used radio broadcasts to urge the remaining residents to leave -- and to warn that anyone "harboring and helping militants will be treated like one of them."

Washington describes eliminating militant havens in Pakistan as vital to turning around the faltering war in Afghanistan. It has strongly backed the military operation and is keen to protect the pro-Western government from the political fallout. The U.N. and international relief groups are also trying to drum up assistance.

U.S. Secretary of State Hillary Rodham Clinton announced Tuesday that Washington would provide $110 million in immediate humanitarian assistance to Pakistan.

"As long as this crisis persists, our assistance will continue," Clinton said. "We know that the work ahead is difficult, but we have seen an enormous amount of support and determination out of the Pakistani government, military and people in the last weeks to tackle the extremist challenge."

As part of that support, two American military planes touched down on Wednesday at an air base near Islamabad laden with air-conditioned tents and 120,000 pre-packed meals, the U.S. Embassy said.

U.S. authorities say they already delivered shipments of wheat and vegetable oil valued at about $28 million to Pakistan last week.