"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.''

"Waves of flood must be met with waves of support from the world"

Ban Ki Moon, UN Secretary General

The floods began more than two weeks ago and have hit about one-quarter of the country. Huge swathes of land remain under water, and hundreds more homes
were flooded over the weekend. Twenty million people are believed to have been affected.

Ban's visit comes a day after the UN confirmed the country's first case of cholera in Mingora, in the northwestern district of Swat, raising the spectre of a disease outbreak over an already bleak situation.

Maurizio Giuliano, spokesman for the UN Office for Humanitarian Affairs, said at least 36,000 people were reportedly suffering from acute watery diarrhoea.

"We're not suggesting that everyone who has acute watery diarrhoea has cholera, but cholera is certainly a concern and that's why we're stepping up our efforts," he said on Saturday.

Relief effort criticised

The United Nations has appealed for $460m to deal with the immediate aftermath of the floods, but charities and officials said the figure falls far short of what is needed.

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"The floods affected some 20 million people, destroyed standing crops and food storages worth billions of dollars, causing colossal loss to national economy," Yousuf Raza Gilani, the Pakistani prime minister, said.

"I would appeal to the world community to extend a helping hand to fight this calamity," he said on Saturday.

Charities said relief for those affected by the disaster, which has left more than 1,600 people dead across the country, was lagging far behind what was needed.

"There are millions of people needing food, clean water and medical care and they need it right now," Jacques de Maio, head of operations for South Asia at the International Committee of the Red Cross (ICRC), said.

"Clearly at this point in time the overall relief effort cannot keep pace with the overall scale of the emergency."

Humanitarian agencies in Pakistan were monitoring the risk of "a second wave of deaths induced by the floods in the shape of water-borne diseases", de Maio said.

Manuel Bessler, the head of the UN office for Co-ordination of Humanitarian Affairs, told Al Jazeera that many parts of the country are only accessible via helicopter, making it difficult to provide drinking water.

"The only means to bring in clean water is tankering, having trucks to reach the area," he said. "But the major challenge is access. In a lot of areas, particularly in the north, we have no access, with all the major bridges that lead over the Swat river washed away."