Villagers use tractors and trucks to cross flood waters in Pakistan's Muzaffargarh district of Punjab province on Monday. (Adrees Latif/Reuters)

Rescue crews in Pakistan unable to reach flood survivors by land dropped food and supplies onto the rooftops of buildings Tuesday to try to bring aid to some of the 20 million people affected by the disaster.

Despite the attempt, and a growing international relief effort, many flood victims have yet to receive any assistance, the United Nations said Tuesday.

The country is reeling under one of its worst-ever natural disasters. About a fifth of the country has been affected since the floods began three weeks ago, including much of its breadbasket in the east.

Prices of food have risen sharply across the country since the floods began, and officials are worried about the prospect of food shortages in the coming months.

Some 6.9 million hectares of farmland in Punjab province are submerged and "completely swollen with water" making feeding people a problem, the CBC's Adrienne Arsenault reported from the city of Multan on Tuesday.

Crop and seed storage facilities are also under water, putting this year's and next year's crops in peril. Cotton crops — a staple of Pakistan's textile industry — that haven't been destroyed can't be processed or manufactured into goods because seven major power plants have been damaged.

Local charities and international agencies have rushed food, water, shelter and medical treatment to the worst-hit areas in the northwest, as well as Punjab and Sindh provinces.

But aid workers are becoming alarmed as flood victims move from region to region trying to evade the flood's path, Arsenault said.

"You have people who have been displaced three or four times each way," she said. "Each time they move, they lose something else. They're not well and they're tired, and it's about 42 degrees here.

"When you see all of the factors together, you start to understand why some of the rescue workers are starting to panic just a little."

Lack of aid criticized

Aid agencies have also complained that the international response to the disaster has not been generous enough.

Pakistani President Asif Ali Zardari acknowledged Tuesday that the government had responded poorly to the widespread flooding. Zardari's reputation sank to new lows after he chose to visit Europe as the crisis was unfolding.

"Yes, the situation could be better. Yes, the arrangements could have been made better. Yes, everything could have been better. Alas! If we could have those resources," he told local aid groups in a meeting. "We have to move forward despite whatever criticism we get."

An aerial view shows the extent of the flooding near the city of Multan, in Punjab province, Pakistan, on Sunday. (Evan Schneider/United Nations/Associated Press)The World Bank said it would provide $900 million US to Pakistan to help it rebuild from the floods. A statement from the bank said the money was being redirected from ongoing and planned projects in the country but did not say whether it was a loan or a gift.

Prime Minister Stephen Harper said Tuesday that Canada has been "generously assisting" the international relief effort in Pakistan, and will continue to do so. Canada has pledged up to $33 million to respond to the "most urgent needs" in flood-affected communities in Pakistan.

Many victims are living in makeshift camps alongside their livestock or in flooded towns and villages.

"The vast geographical extent of the floods and affected populations meant that many people have yet to be reached with the assistance they desperately need," the UN said in a statement.

The world body also said the number of children and breast-feeding mothers affected and rising diarrhea cases "point toward a clear risk of malnutrition among the affected population."

Authorities in Sindh province said more floods were likely over the next 24 to 48 hours. The swollen Indus River may burst its banks again in coming days.

"The next two days are crucial for the safety of people," said Sindh's irrigation minister, Jam Saifullah Dharejo.

With files from the CBC's Adrienne Arsenault and The Associated Press