PESHAWAR, Pakistan (AFP) – Pakistan fighter jets and attack helicopters pounded Taliban targets in the northwest on Wednesday as President Asif Ali Zardari called for global help to avert a humanitarian catastrophe.

Hundreds of thousands of civilians have fled the punishing offensive in the Swat valley, escaping Taliban fighters who have terrorised the population in a bloody campaign to enforce sharia law and expand their control.

Terrified residents trapped in Mingora, the district's main town, told AFP by telephone that militants had planted mines and were digging trenches.

"Please, please, please, do not call me again, they will cut my throat and say that I was spying," said one resident contacted by AFP, pleading for his name not to be published.

"People are becoming mentally ill, our senses have shut down, children and woman are crying, please tell the government to pull us out of here," said a shopkeeper, also on condition of anonymity.

"Just imagine how we are surviving. Forget the lack of electricity and other problems, the Taliban are everywhere and heavy exchanges of fire are routine at night."

Air strikes targeted Taliban strongholds across Swat, which has sunk from stunning ski resort favoured by Westerners to crucible of Taliban violence and where ground troops have yet to take control.

Helicopter gunships also swung into action in the neighbouring district of Lower Dir, where the military has been on the offensive since April 26 after Taliban fighters advanced to within 100 kilometres (60 miles) of Islamabad.

Up to 15,000 security forces are taking on about 4,000 well-armed fighters in Swat in what Islamabad says is a battle to "eliminate" Islamist militants, branded by Washington as the greatest terror threat to the West.

Military officials said exit roads from Mingora had been closed and troops were surrounding the town to prevent militants leaving.

Amjad Ali, a 35-year-old plumber, said he and his four children walked for three days to the Jalozai refugee camp to escape scenes of horror in Mingora, where Taliban were armed with guns, sniper rifles and rocket launchers.

"Bodies were dragged by dogs... nobody could collect them," he said.

"There was a lot of fear because they (the Taliban) are famous for killing and beheading the people," he said.

Zardari, who has been in the United States for talks with President Barack Obama, met UN Secretary General Ban Ki-moon in New York, where he appealed for global aid for the displaced.

"We're appealing to the world, myself and the secretary general... to draw attention on the human catastrophe that is taking place," he told reporters.

Ban expressed "deep concern" about the humanitarian situation in the area, where the UN refugee agency said 501,496 stranded people had registered with authorities since May 2.

Even before the latest fighting, the United Nations was dealing with about half a million displaced people in the country.

Obama's administration, which has put Pakistan at the heart of its fight against Al-Qaeda, is also lobbying that for a giant aid package, arguing that to abandon the nuclear-armed nation would risk vital US national interests.

Airborne commandos Tuesday opened a new front in Swat's northern mountains, suspected stronghold of firebrand Taliban commander Maulana Fazlullah and top aides behind a nearly two-year uprising that has devastated the area.

Overall, the military says 751 militants and 29 troops have been killed in its operations in Lower Dir, Buner and Swat, although there is no independent confirmation of the figures and no word on civilian casualties.

Pakistani troops have conducted operations against extremists in parts of North West Frontier Province over the past two years, and for around six years in the surrounding semi-autonomous tribal belt on the Afghan border.

Extremist attacks have killed at least 1,800 people across Pakistan in less than two years while around 2,000 soldiers have died in battles with Islamist militants since 2002.