SUKKUR, Pakistan – The deadly, waterborne disease cholera has surfaced in flood-ravaged Pakistan, the U.N. confirmed Saturday, adding to the misery of 20 million people the government says have been made homeless by the disaster. A fresh surge of floodwater swelled the Indus River, threatening previously spared cities and towns in the south.

The crisis has battered Pakistan’s economy and undermined its political stability at a time when the United States needs its steadfast cooperation against Islamist extremism. The U.N. has appealed for an initial $460 million to provide relief to Pakistan but has said the country will need billions to rebuild once the floodwaters recede.

Because of the flooding, Pakistan canceled celebrations Saturday marking its creation and independence from Britain in 1947. President Asif Ali Zardari met with flood victims in the northwest, and U.N. Secretary-General Ban Ki-moon was expected to visit affected regions today.

The floods have killed about 1,500 people, and aid workers have warned that diseases could raise that toll.

One case of cholera was confirmed in Mingora, the main town in the northwest’s Swat Valley, U.N. spokesman Maurizio Giuliano said Saturday. But other cases were suspected, and aid workers are now responding to all those exhibiting acute watery diarrhea as if it is cholera, Giuliano said.

Cholera can lead to severe dehydration and death without prompt treatment, and containing cholera outbreaks is considered a high priority following floods.

The Pakistani crisis began in late July, when unusually heavy monsoon rains tore through the country from its mountainous northwest. Hundreds of thousands of homes have been destroyed. Agriculture has been severely hit, with an estimated 1.7 million acres of farmland wiped out.

U.N. officials, citing government figures, previously said about 14 million Pakistanis were directly or indirectly affected.

But in a televised address to the nation Saturday, Prime Minister Yousuf Raza Gilani said 20 million were now homeless. He did not elaborate, and it was unclear how many of those people had been briefly forced to leave their homes and how many had lost their houses altogether.

Fresh flood waves swelled the River Indus on Saturday, threatening nearby cities, towns and villages in southern Sindh province, said Mohammed Ajmal Shad, a senior meteorologist. The Indus was already more than 15 miles wide at some points Friday — 25 times wider than during normal monsoon seasons.In a televised address Saturday, Prime Minister Yousuf Raza Gilani said 20 million were now homeless.