Three weeks after monsoon rains triggered the first flooding in Pakistan, a fifth of the country's land mass is now submerged.
Secretary-General Ban Ki-moon has told an emergency session of the United Nations General Assembly the floods are like a slow-motion tsunami.
An estimated 20 million people are affected and aid officials say shelter, food and clean water are urgently needed to avert a humanitarian catastrophe.
Pakistan's Foreign Minister Shah Mahmood Qureshi said he had been assured that the UN's target of $US460m would be "easily met", the BBC reports.
The government, the UN and aid agencies say while they have been able to reach more flood victims, a shortage of funding has prevented them delivering the volume of relief supplies that are needed.
In the province of Sindh and parts of Punjab, the waters are continuing to rise - meaning more people on the move and many without access to clean water and vital medical care in particular.
The number of people in need of immediate assistance has now risen to eight million.
Meanwhile, aid agencies in New Zealand are pleading with the public to be more generous in their donations for the flood victims
Several aid agencies say donations are only trickling in and they have collected less than $NZ500,000.
The Asian Development Bank is to offer Pakistan an emergency loan of $US billion to help repair the devastation.