Islamabad - Pakistani forces expanded their offensive in the Taliban stronghold of Swat on Saturday as a humanitarian crisis intensified with thousands of civilians marooned on the battlefield.

The United Nationals refugee agency estimated up to 200,000 people fled the former tourists' haven in recent days, but forecast further exodus of nearly 300,000 more.

Pakistani military announced "a full-scale operation" Friday, the day after Prime Minister Yousuf Raza Gilani called for national unity as the troops were ordered "to eliminate the terrorists."

United States welcomed the military action amid concerns for the dwindling stability in the nuclear-armed state, which it considers central to the counter-insurgency efforts in Afghanistan.

Helicopter gunships pounded militant positions in Swat's main town of Mingora Saturday to soften Taliban resistance as troops pushed forward with the ground operation, a local official said on condition of anonymity.

There were no immediate reports of casualties but several militant hideouts were destroyed in the airborne attacks.

Clashes were also reported in other parts of Swat and the adjoining district of Buner and Lower Dir, but details were not available.

More than 140 Taliban fighters were reported killed on the first day of the all-out offensive, with the troops also suffering at least seven casualties. The toll could not be verified independently.

Military spokesman Major General Athar Abbas told reporters that troops were battling up to 5,000 hardcore Taliban militants in the scenic valley, located 140 kilometers north-west of the capital Islamabad.

Officials said the militants were using the residents as "human shield" and preventing them from fleeing the escalating hostility.

An indefinite curfew remained in place in the insurgency-hit areas of Swat, with the authorities saying that the relaxation timings could not be disclosed in advance because militants were ought to attack non-combatants leaving the valley.

Those taking chances also faced the treat of being caught in crossfire or hit by roadside bombs planted by the insurgents.

Commentators say there is a wide consensus, at least for now, that the government needs to go all-out against the Taliban, but warn that the situation could change in case of heavy collateral damage and improper rehabilitation of the displaced people.

Pakistani authorities have set up nearly a dozen camps with support from international and local relief agencies. However, UN refugee agency spokesman in Geneva, Ron Redmond, said the new influx would place "huge additional pressure on resources." (dpa)