UNITED NATIONS - The United Nations appealed for nearly $460 million Wednesday to provide immediate help to millions of victims of the worst floods in Pakistan in living memory and said it will need millions more to help rebuild the devastated country.

"Make no mistake, this is a major catastrophe," U.N. humanitarian chief John Holmes told diplomats from several dozen countries in launching the appeal. "The affected population is estimated to be more than 14 million - almost one-tenth of Pakistan's population."

The U.N. Office for the Coordination of Humanitarian Affairs, which Holmes heads, said at least six or seven million flood victims require immediate humanitarian assistance including shelter, food, clean water, sanitation and medical care.

Before the $459.7 million appeal was launched, the U.N. already had pledges and commitments of more than $150 million but some $300 million is still needed, Holmes said.

He told reporters after Wednesday's pledging conference that he was encouraged by the turnout and generosity of donors, and the commitment of many countries to continue helping Pakistan.

The United States made the biggest pledge, $71 million. Britain said it has given 5 million pounds ($7.8 million) to the U.N. children's agency, UNICEF, for water and sanitation; $7.8 million to the Pakistani Emergency Response Fund; $15.6 million to rebuild bridges and $1.2 million for emergency seed money as well as 2,500 tents, and $7,000 for Pakistan's emergency radio broadcast program.

Other donations included $13 million from Germany, $10 million from Australia, $5 million from Kuwait, $3.5 million from Japan and $3.3 million from Norway.

U.S. military helicopters and Pakistani military have rescued approximately 2,300 people and transported over 200,000 pounds of relief supplies.

The U.S. aim is to support Pakistan's relief effort "to get aid as rapidly as possible to those in need," she said.

"We have a huge task in front of us," Holmes said. "The death toll has so far been relatively low compared to other major natural disasters, but the numbers affected are extraordinarily high. If we don't act fast enough, many more people could die of diseases and food shortages. ... The incidence of acute diarrhea and other waterborne disease is rising worryingly."

According to the U.N., over 1,200 people have died and at least 288,000 homes have been damaged or destroyed in the flooding.

Pakistan's U.N. Ambassador Abdullah Haroon called the floods a particularly "savage attack by nature" and praised the United Nations for "assisting in every way possible."

But he said more help was needed and appealed especially for clean water to prevent cholera and assistance in dealing with snakebites, saying thousands of people have already been bitten.

Haroon said the number of people affected could rise further because the floods are spreading and a week of further rains is predicted. He worried that as many as 16 million could be affected.

Noting that the Asian tsunami in 2004 displaced less than 2 million people, Haroon said, "This is seven times the amount. This is horrendous."