"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."
Holmes said the money sought represented the minimum needed for emergency assistance over the next three months and did not include anything for rebuilding infrastructure.
Fields swept away
At least 1.4 million acres of crops were destroyed in Punjab, the breadbasket of Pakistan, according to the United Nations. Fields in the floodplains of the northwest have also been devastated as rivers have spilled over their banks.
An official in Pakistan's northwestern province of Khyber Pakhtunkhwa said maize, rice, sugarcane and vegetable crops were the most affected.
"I had 200 kilos of corn at my home which the flood took away with it," Dil Aram Khan, a farmer from Pirpai in Nowshera District, said.
"All of our wealth is destroyed along with our wheat."
At least four million people are expected to need food assistance across Pakistan for the next three months at a cost of nearly $100m.
Marcus Prior, the spokesperson for the World Food Programme (WFP), said his organisation had received a "pretty decent response so far".
"We've had just over $13m committed and we have good indication that other donors are going to step in and help us out in a very significant way in the days ahead," he told Al Jazeera.
"The fact is that we have at least the food we need for the first month to feed those people who have been so badly affected ... But we need contributions to buy food locally here in Pakistan."
Food prices soar
Food prices shot up significantly as the Muslim fasting month of Ramadan started and are likely to exercabate misery in a country where many people live in abject poverty.
The prices of basic items such as tomatoes, onions, potatoes and squash have in some cases quadrupled in recent days, putting them out of reach for many Pakistanis.
||Four million left homeless
||13.8 million displaced or affected
||558,000 hectares of farmland under water
"We will fast but we don't know how will we break the fast, whether we will find any food or not. Only Allah knows," Nusrat Shah, a displaced Pakistani in Sukkur, told the Reuters news agency.
Al Jazeera's Sohail Rahman, reporting from Swat Valley, one of the worst-hit areas, said it was unclear whether the situation might actually get worse before it gets better.
"We're still in the middle of the monsoon season ... the clouds come and go," he said.
"They certainly did on Tuesday [and] it rained all through the night and then it cleared on Wednesday. Of course, no one knows what will happen."
US army helicopters have been carrying emergency food and water to areas in the northwest.
But our correspondent said "whatever aid does comes in it could not reach some people too soon".
Jane Cocking, the humanitarian director at Oxfam, said "people are absolutely desperate".
"We are fixing water supplies, distributing really essential items, but at the moment it is not enough. Any new donation is welcome but to be perfectly honest it is not enough yet," she told Al Jazeera.
"The scale of the disaster is almost impossible to describe. And I think that might be why people are struggling to understand it, but we're talking about half of Pakistan which is an enormous country and that's absolutely massive."
Meanwhile, Pakistan's Taliban has denounced all foreign aid for flood victims and said they can match the latest US pledge of $20 million.
"We condemn American and other foreign aid and believe that it will lead to subjugation. Our jihad against America will continue," Azam Tariq, a spokesman for the group, told the AFP news agency.
"The government should not accept American aid and if it happens, we can give $20m to them as aid for the flood victims," he said.
"We'll ourselves distribute relief under leadership of our chief Hakimullah Mehsud among the people, if the government assures us that none of our members will be arrested."