Pakistani flood survivors eat in a camp set up for displaced people in Sukkur, Pakistan. (Shakil Adil/Associated Press)

UN Secretary General Ban Ki-moon said Sunday the flooding in Pakistan is the worst disaster he has ever seen, and he urged the world to do more for the flood-ravaged country.

"This has been a heart-wrenching day for me," he said after flying over some of the worst-hit areas.

"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."

The UN chief also pleaded for countries to speed up their assistance to the Pakistani people.

The UN has issued a global appeal for $460 million US in immediate help.

Canada announced Saturday it would give $33 million in humanitarian aid.

New flooding

The UN chief's visit came as new flooding in southern Sindh province threatened to worsen an already critical situation.

The Indus River and other waterways were registering more flood surges. Authorities said water levels weren't expected to peak until later Sunday.

The CBC's Adrienne Arsenault, reporting from Pakistan's Swat Valley, said the power of the flood waters is much in evidence.

"We saw places where there used to be cornfields and schools and roads and now it's just a river bed," she said.

"We spoke to mathematicians and engineers and principals who said, 'We were successful business people and we literally have nothing.'"

Arsenault reported that the Canadian Red Cross has been busy handing out mosquito nets, buckets and tarps to people who, in some instances, have walked six or seven hours from their villages to collect the supplies and must now walk six or seven hours back to their ruined towns.

Up to 20 million homeless

Floods, triggered by heavier than normal monsoon rains, have killed more than 1,500 people in the last couple of weeks and left 20 million homeless, according to government estimates.

Many people have diarrhea. Cholera and malaria are growing concerns, said Matt Capobianco, an emergency planning manager for the Canadian charity GlobalMedic.

His organization has set up aid stations in northwest Pakistan that are providing clean drinking water for 15,000 people a day.

But Capobianco said aid agencies are still struggling to get to many victims.

"The scope [of the disaster] is absolutely enormous," he told CBC News on Sunday. "With the amount of people that are affected here, there are definitely many falling though the cracks."

With files from The Associated Press