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

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

“This has been a heart-wrenching day for me,” Ban said Sunday 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’s Sichuan province 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 northwest have now hit about one-quarter of the country, especially its agricultural heartland. While the death toll of 1,500 is relatively small, the scale of the flooding and number of people whose lives have been disrupted is staggering.