Some 20 million flood victims in Pakistan have yet to receive any aid despite a growing international relief effort, the UN said Tuesday.

The country is dealing with one of the worst natural disasters in its history, testing a shaky government that is vital to the West's war against al Qaeda and the Taliban.

Michael O'Brien with the International Red Cross says the problem of aid is both one of supply and distribution.

"The task at hand with the relief operation is so substantial that we are really just focused on getting life-saving aid to the people that need it the most," he told CTV from Islamabad.

"It will take years for this country to recover."

Aid from both local and international agencies has been rushed to the worst-hit areas in the northwest and Punjab and Sindh provinces.

But aid agencies and the British government say not enough international aid has been sent.

The World Bank announced Tuesday it was redirecting US$900 million of its loan to Pakistan to help in the flood recovery.

The UN asked for US$459 million in for immediate aid last week. It says it has received 40 per cent -- about $184 million -- of that figure so far.

Many victims are living in hastily-assembled camps with their livestock.

"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.

At least 1,500 people have died in the flooding.

There are worries in Sindh province that more flooding is on its way in the next 48 hours.

The disaster began to unfold about three weeks ago and it's expected that the situation could deteriorate over the next month as the Indus River continues to swell.

About a fifth of the country is underwater, shaking the country's weak economy to its core.

Local food prices are rising, as 700,000 hectares of wheat, sugar cane and rice sit rotting under the water.

"It's all been destroyed. I'll get nothing this year," wheat farmer Zubair Ahmed told The Associated Press. "Nobody cares in the government. We're stranded, and they don't even pay attention to us."

President Asif Ali Zadari, meanwhile, has faced heavy criticism for inaction during the disaster. In fact, he was in Europe on a state visit when the crisis began.

"Yes, the situation could have been better. Yes, the arrangements could have been made better. Yes, everything could have been better," he said. "We have to move forward despite whatever criticism we get."

In the hard-hit Sindh province, aid was nearly non-existent.

"We have never dealt with a calamity of this magnitude," said Faisal Edhi, who works for the Edhi Foundation, an aid group. "We're trying to make it, but I don't think we've been able to help 20 percent of the people we want to."

With reports from The Associated Press