"We need a lot more, and we need it quickly," John Holmes, the humanitarian coordinator for the UN, said.
The UN children's fund (Unicef), meanwhile, said that funding shortfalls could mean slower assistance for the more than three million children affected by the flooding.
"We cannot spend pledges. We cannot buy purification tablets, we cannot support Pakistan with pledges," Daniel Toole, the South Asia regional director for Unicef, said. "I urge the international community to urgently change pledges into cheques."
The UN has warned that up to 3.5 million children could be in danger of contracting deadly diseases carried through contaminated water and insects.
Ted Itani, from the International Red Cross and Red Crescent, operating in Pakistan, told Al Jazeera that the organisation cannot deal with the fallout from the flooding, let alone a pending second disaster caused by the outbreak of disease.
"I am hampered by [lack of] access to the beneficiaries I am mandated to serve, as well as information. We need more timely and accurate information [to do our job]," Itani said.
"And thirdly there are financial constraints because in our case I can only spend cash that is in my budget. Although donors have pledged millions of dollars it has to filter down into my account so I can order things before the onset of winter.
"Many of these commodities take weeks of lead time to get them into the system. We are counting on the generosity of ordinary people to donate money to this cause."
'Billions' in costs
The World Bank on Monday announced that it will make $900 million in loans available for relief efforts.
A spokesman for the bank said those funds will come through the reprogramming of planned projects, and the reallocation of money.
The organisation, along with the Asian Development Bank and the United Nations, have been asked to conduct assessments of the flood-hit areas in Pakistan.
Several other countries have announced major donations over the last few days. Saudi Arabia said on Tuesday that it had raised more than $20 million in the first day of a national fundraising campaign. Japan pledged an additional $10 million; Australia promised more than $21 million.
Wajid Shamsul Hasan, Pakistan's high commissioner to the United Kingdom, said on Monday that the cost of rebuilding his country could exceed $10 to $15 billion.
'Fake camp' erected
Millions of victims continue to complain about inadequate aid from the Pakistani government.
At least 1,500 people killed 20 million people affected 6 million people estimated to need food aid 722,000 homes damaged or destroyed 700,000 hectares (1.7 million acres) inundated $459m needed to deal with immediate problems $184m received so far
In Sindh province on Tuesday, hundreds of people mobbed two food delivery trucks; aid workers lashed at them with ropes to keep them away.
Yousuf Raza Gilani, the Pakistani prime minister, visited a "relief camp" near Dera Ismail Khan in Khyber-Pakhtunkhwa province.
But Pakistani media reported that the "camp" was erected just hours before Gilani arrived, and that it was "wound up soon after [his] departure". Residents of the camp told the Dawn newspaper that they have been living out in the open, with no shelter.
"At times there is no food and we starve," an old man named Mohammad Shafi told the newspaper.
Gilani is traveling on Wednesday in the Swat valley in northwest Pakistan, which has seen some of the most extensive damage from the flooding.
Asif Ali Zardari, the Pakistani president, has admitted that the government's response to the disaster has been inadequate.
"Yes, the situation could have been better. Yes, the arrangements could have been made better. Yes, everything could have been better," Zardari acknowledged.