Pakistan fighter jets and attack helicopters pounded Taliban hideouts in the northwest on Wednesday as President Asif Ali Zardari appealed for global help to avert a humanitarian catastrophe. Hundreds of thousands of civilians have fled the punishing offensive, now into its 17th day, escaping also out of the reach of Taliban fighters who have terrorised the local population in a bloody campaign to enforce sharia law. The air attacks targeted Taliban strongholds across the iconic Swat valley, which has sunk from stunning ski resort favoured by Westerners to crucible of Taliban violence, security officials said. 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 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 calls a battle to "eliminate" Islamist militants, branded by Washington as the greatest terror threat to the West. "All exit roads from Mingora have been closed. Our troops have surrounded the city to deny any exit to militants," a military official, referring to the main town in Swat. "We have also blocked the road to Dir as militants were using the route to flee to the tribal district of Bajaur," the official 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. "They (civilians) are losing their crops, they're losing their earnings, their livelihood and their homes, so we want the world to help us." 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. Ban said the United Nations "is ready to stand by to provide the necessary humanitarian assistance." UN humanitarian chief John Holmes promised to increase "substantially" an aid appeal for Pakistan -- having already requested 165.9 million dollars for humanitarian aid in the country for 2008-2009. 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, also lobbied hard for a giant aid package, arguing that to abandon the nuclear-armed nation would risk vital US national interests.