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, 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 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.

Australia's ABC news reported that Ban was visibly shaken:

"The magnitude of the problem; the world has never seen such a disaster. It's much beyond anybody's imagination," he said. "This is a long-term affair; this is a two-year campaign. We have to consider that and keep that in mind. "For two years we've got to give them crops, fertilisers; we've got to give them seed; we've got to look after them, feed them, for two years, to bring them back to where they were. And they will still not be where they were."

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. 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.

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 people. The International Monetary Fund has warned the floods could dent economic growth and fuel inflation.

"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," he said.

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

In his first comments to the media since returning, he defended the government.

"The government has responded very responsibly," he said, saying the army, the police, the navy and officials were all working to relieve the suffering. "I would appeal to the press to understand the magnitude of the disaster."

Zardari said it would take up to two years for the country to recover.

Ban said visa restrictions had been eased for humanitarian workers and they now could get visas on arrival at Pakistan airports.

On Saturday, the prime minister said 20 million people had been made homeless in the disaster.

The monsoon rains that triggered the disaster are forecast to fall for several weeks yet, meaning the worst may not yet be over. Over the weekend, tens of thousand of people were forced to flee their homes when they were inundated by fresh floods from the swollen River Indus.

While local charities and international agencies have helped hundreds of thousands of people with food, water, shelter and medical treatment, the scale of the disaster has meant that many millions have received little or no assistance. The U.N. has voiced fears that disease in overcrowded and unsanitary relief camps may yet cause more deaths.

Earlier Sunday, survivors fought over food being handed out from a relief vehicle close to the town of Sukkur in hard-hit Sindh province, ripping at each others' clothes and causing such chaos that the distribution had to be abandoned, according to an Associated Press reporter at the scene.

"The impatience of the people has deprived us of the little food that had come," said Shaukat Ali, a flood victim waiting for food.

Waters five feet (1.5 meters) deep washed through Derra Allah Yar, a city of 300,000 people on the border of Sindh and Baluchistan provinces, said government official Salim Khoso. About 200,000 had fled the city and Khoso said he did not know how they would be fed.

"We are here like beggars," said Mukhtar Ali, a 45-year-old accountant living on the side of a highway along with thousands of other people. "The last food we received was a small packet of rice yesterday and 15 of us shared that."