Louis-George Arsenault, director of emergency operations for Unicef in New York, described the lack of support as "quite extraordinary".
The humanitarian crisis was the largest "in decades", he warned.
Mr Arsenault spoke as the International Monetary Fund was due to start talks with Pakistani officials in Washington.
The talks on Monday will allow the IMF to assess how best to give help. It says the floods that have struck Pakistan pose a "massive economic challenge" and it will review the country's budget and financial prospects.
The UN says it has now raised close to 70% of the $460m it called for in its emergency appeal, as donors pledged more money. Some $54m are in uncommitted pledges, and $263m are resources available now.
Pakistan has also accepted $5m (£3.2m) in aid from its rival and neighbour India.
In southern Pakistan, tens of thousands of people are fleeing a threatened flood surge, three weeks after heavy monsoon rains first hit the country.
A hastily built barrier has been breached in the city of Shahdadkot, allowing floodwaters to approach houses.Millions displaced
As officials prepared for the meetings, Mr Arsenault, of the UN children's fund, said: "One of the major challenges that we have which is quite extraordinary is the lack of level of support from the international community.
Tens of thousands more Pakistanis have been fleeing the floods, with the south now bearing the brunt.
Overall, about 1,600 people have been killed and some 16.8 million affected, according to figures from the UN and Pakistani government.
In the city of Shahdadkot, a hastily built barrier has been breached, allowing floodwaters to approach houses.
An estimated four million people have now been displaced in the city of Sukkur alone.
Meanwhile, the World Health Organization (WHO) says diseases are spreading in affected areas.'Unprecedented flood'
Sindh, in the south of Pakistan, is now being described as the country's worst-hit province, with officials saying at least 200,000 residents have fled in the last 24 hours.
In Shahdadkot, the BBC's Jill McGivering says residents are leaving en masse to try to reach safe ground.
The makeshift 4ft mud barrier, built by the army and volunteers, was the city's last line of defence and has now been breached in several places.
Jam Saifullah Dharejo, Sindh provincial irrigation minister, said that most people had been escorted to safety, but efforts were under way to help those still stranded.
"We are trying to save the city from the unprecedented flood," he said.
The Pakistan government has said that the cost of rebuilding after the floods could be as high as $15bn (£10bn).
About one-tenth of the homeless have places in relief camps, the rest are trying to survive alone, without shelter or any assurance of food, she says. Aid is being provided but it is limited and in enormous demand.
Dozens more villages have been inundated and although authorities expect flood waters to drain into the Arabian Sea over the next few days, evacuees who return may find their homes and livelihoods have been washed away.
The floods began last month in Pakistan's north-west after heavy monsoon rains and have since swept south, swamping thousands of towns and villages in Punjab and Sindh provinces.
The UN said on Friday that more helicopters were urgently needed to reach communities cut off by the water.
Experts warn of a second wave of deaths from water-borne diseases such as cholera unless flood victims have access to supplies of fresh drinking water.
If you would like to make a donation to help people affected by the floods in Pakistan, you can do so through the UK's Disasters Emergency Committee at www.dec.org.uk or by telephone on 0370 60 60 900.