Torrential monsoon rains unleashed the worst floods in 80 years, flattening houses, sweeping away transport and communication links as well as causing destroying livestock and crops.
Rebecca Wynn of the UK-based aid agency Oxfam told al Jazeera that the floods were an "unprecedented disaster" and further pledges would be needed to meet the victims' needs.
"The figures keep going up. First it was two million [people affected], then it was four million, and now it is 20 million. It's shocking," she said.
"The [UN] appeal is just for the first 90 days and it was based on an estimate of six million people, so more money will be needed."
Eight million flood survivors in desperate need of food, shelter and clean drinking water require humanitarian assistance to survive, as concerns grow over potential cholera, typhoid and hepatitis outbreaks.
"The situation is very serious and is growing. The worse has to come," Guido Sabatinelli, the World Health Organisation representative in Pakistan, said.
"We are receiving some good pledges. We cannot buy drugs with the pledges. We need to convert this into cheques."
Sabatinelli said that there had been a 30 per cent rise in diarrhoea case in the flood-affected areas.
Elisabeth Byrs, a spokeswoman for the UN Office for the Co-ordination of Humanitarian Affairs (OCHA), said that a full picture of the true scale of the disaster was only just beginning to emerge.
"It's a disaster that came very slowly, it's not an earthquake that hits suddenly that we can immediately see the victims. But we are now seeing the magnitude of this catastrophe."
Last week, OCHA launched an emergency appeal for $460 million for Pakistan and on Friday it said 55 percent of the requested funds had been received.
The General Assembly meeting on the Pakistan floods was continuing on Friday, with more speakers offering pledges.
India, which has fought three wars with its neighbour, was among those pledging aid, with a $5m donation accepted by Islamabad.
The official death toll from the floods is about 1,500, but Pakistan's envoy to the UN said on Friday that the final toll could be much higher.
"We don't yet know how many are dead and how many have perished," Abdullah Hussain Haroon told the UN General Assembly.
"We can only hold our breath and hope that the casualty figures have been fewer."
The floods began on July 29 in the northwest of the country after exceptionally heavy monsoon rains and have since swamped thousands of towns and villages in Punjab and Sindh provinces.
While rainfall has lessened, flooding is continuing in parts of Sindh province as water from the north courses down the Indus and other rivers.
The Nato military alliance agreed on Friday to provide an airlift and sealift for aid to Pakistan and said the first cargo plane would fly there at the weekend with power generators, water pumps and tents.
Many areas are inaccessible by road and the only way to provide relief to them is through helicopters.