ISLAMABAD, Pakistan – U.N. Secretary-General Ban Ki-moon said Sunday he has never seen anything like the flood disaster in Pakistan after surveying the devastation and urged foreign donors to speed up assistance to the 20 million people affected.

Ban’s comments reflect the concern of the international community about the unfolding disaster in Pakistan, which is battling al-Qaida and Taliban militants and has a weak and unpopular government, as well as an anemic economy propped up by international assistance.

“This has been a heart-wrenching day for me,” Ban said after flying over the hard-hit areas with President Asif Ali Zardari. “I will never forget the destruction and suffering I have witnessed today. In the past I have witnessed many natural disasters around the world, but nothing like this.”

Ban visited Myanmar after Cyclone Nargis devastated the country in May 2008, killing an estimated 138,000 people. He also flew to China just days after an earthquake killed nearly 90,000 people in March 2008.

The floods that began more than two weeks ago in Pakistan’s mountainous northwest have now hit about one-quarter of the country, especially its agricultural heartland. The death toll of 1,500 is relatively small, but the scale of the flooding and number of people whose lives have been disrupted are staggering.

The world body has appealed for an initial $460 million to provide relief, but only 20 percent has been given.

Once the floods recede, billions more will be needed for reconstruction and getting people back to work in the already-poor nation of 170 million. The floods could dent economic growth and fuel inflation, the International Monetary Fund has warned.

“Waves of flood must be met with waves of support from the world,” said Ban. “I’m here to urge the world to step up assistance.”

Zardari has been criticized for his response to the disaster, especially for going ahead with a state visit to Europe as the crisis was unfolding. He has visited victims twice since returning, but images of him at a family-owned chateau in France are likely to hurt him for months.

In his first comments to the media since returning, he said, “The government has responded very responsibly.”

Prime Minister Syed Yousaf Raza Gillani said Saturday that 20 million people are homeless.