Launching the UN's appeal in New York, emergency relief coordinator John Holmes said the funds would be used for food, clean water, shelter and medical supplies.
"The death toll has so far been relatively low compared to other major natural disasters, but the numbers affected are extraordinarily high," Holmes warned.
"If we don't act fast enough, many more people could die of diseases and food shortages."
The UN says that children are among the most vulnerable, with diarrhoea the biggest health threat and measles a concern.
Governments have so far pledged 117 million euros in emergency assistance.
Holmes stressed that 460 million dollars was the minimum required to provide emergency relief over the next three months, and would not cover the cost of rebuilding Pakistan's infrastructure destroyed in the floods.
Pakistan's government has called the flood the worst natural disaster in the country’s history.