People flee their flooded villages in Rajanpur in Pakistan on Sunday. (Shiekh Saleem Raza/Associated Press)

Heavy rains are making it even more difficult to get help to the millions of Pakistanis affected by the worst floods to hit the country in decades.

Chopper flights that were to deliver supplies to cut-off communities in the Swat Valley were grounded Sunday because of poor visibility.

The World Food Programme says as many as four million Pakistanis face food shortages after floods destroyed up to 570,000 hectares of crops in central Punjab province alone, according to UN estimates.

Prices for many fruits and vegetables were reportedly soaring throughout Pakistan on Sunday.

Pakistani Prime Minister Yousuf Gaza Gilani issued an appeal for more international aid, saying the damage from these floods was "far greater" than the earthquake that killed 73,000 in Kashmir in 2005.

"That's why we are trying to contact the world to help us," Gilani said as he toured an area where the Indus river overflowed its banks in southern Sindh province, destroying 1,500 mud homes in one village alone.

"The losses we have had are irreparable," he said.

Canada has pledged $2 million in aid for the country. Many private fundraising efforts are also underway in Canada.

Billions to recover

The UN special envoy to Pakistan estimated it would take billions of dollars for Pakistan to recover and rebuild once the waters recede.

It's estimated that weeks of flooding have killed more than 1,600 people and severely disrupted the lives of 15 million. More than 250,000 homes have been destroyed or damaged.

The death toll includes at least 53 people killed Saturday when heavy rains triggered landslides that buried two villages in northern Pakistan.

"We are sitting on the bank with nothing in our hands," said Allah Bux, a flood victim in Sukkur. "No shelter, no food. We are helpless."

Thousands of others in Sindh province have fled.

With files from The Associated Press