United Nations Secretary General Ban Ki-moon has said the Pakistan flooding is the worst natural disaster he has ever seen - as more heavy rain heads towards the worst-hit areas.

With the first case of cholera reported, Ban has also urged foreign donors to speed up aid to the country.

The World Health Organisation (WHO) has said as many as 300,000 people may contract cholera and up to seven million will suffer from diarrhoea.

Up to 1,600 people have died in the disaster and around 20 million have been left homeless.

After pledging a further $10m from the UN's central emergency response fund, Ban said: "This has been a heart-wrenching day - I will never forget the destruction and suffering I have witnessed.

Aid is dropped from helicopter to flood victims in Punjab province

"In the past I have witnessed many natural disasters around the world, but nothing like this.

"I am here... to share my sympathy and solidarity of the United Nations together with the people and government of Pakistan at this time of trial.

"I am here also to urge the world community to speed up their assistance to Pakistan.

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

So far, only one case of cholera has been confirmed by the UN, but other cases are suspected among those with diarrhoea.

Mark Ward, acting director of the US department for foreign disaster assistance, said cholera was "unavoidable" - but could be controlled.

He praised a system set up by the WHO to quickly detect any cases of the disease or other waterborne illnesses common after flooding.

"The good news is that we know where it is and we can get resources in there to help because of the disease early warning system," he said.

"When you are dealing with this much water and that many people, it (cholera) is almost unavoidable. I think we can control this."

Cholera, a bacterial intestinal infection typically spread through contaminated water, causes severe diarrhoea and dehydration and can be fatal.

Flood victims evacuate their villages in Sukkur, Sindh province

Fresh flood waves have swelled the River Indus, threatening nearby cities, towns and villages in southern Sindh province, said senior meteorologist Mohammed Ajmal Shad.

Authorities are trying to evacuate or warn people in Jacobabad, Hyderabad, Thatta, Ghotki, Larkana and other areas in Sindh province that so far have been spared floods.

Meanwhile, more American helicopters have joined food aid work, with the US assigning aircraft from the USS Peleliu in the Arabian Sea.

And France has announced it will send a plane loaded with 60 tons of aid to the region.

:: Visit the Disasters Emergency Committee's website (http://www.dec.org.uk/donate_now/) for details on how to donate to Pakistan floods appeal or call 0370 6060 900.