The UN has launched an appeal for $459m (£290m) to help victims of Pakistan's flood disaster, which has affected at least 14 million people.
UN humanitarian chief John Holmes said the money would be for immediate relief over the next three months.
The disaster was "one of the most challenging that any country has faced in recent years", he said in New York.
So far, about 1,600 people have been killed by the monsoon floods.
Mr Holmes said that unless the provision of aid such as food and clean water to millions of displaced people was rapidly increased, many more lives would be at risk.
Amid concern that militant groups may strengthen their control of areas of Pakistan by handing out aid of their own, Mr Holmes added that the appeal was about relieving people's suffering, not politics.
"Others may be concerned about that, but that cannot be the concern of the UN and the UN agencies and the humanitarian agencies," he said.
Weather officials say the monsoon system over the Arabian Sea is weakening and there should be a break in the torrential rains for at least the next three days, though scattered rains are forecast.
The UK responded to the UN appeal by announcing it would supply aid to some 1.5 million people in Pakistan.
Four further plane loads of relief supplies will be sent, said the Secretary of State for International Development, Andrew Mitchell.
The US has deployed a carrier to Pakistan that will triple the number of American helicopters available for use in the flood relief effort, Defense Secretary Robert Gates was quoted as saying in Florida by AFP news agency.
US helicopters have already been helping in northern parts of Khyber Pakhtunkhwa province.
It has been described as one of the biggest appeals in the UN's history in terms of the number of people who are in need, the BBC's Lyse Doucet reports from the Pakistani capital Islamabad.
With the monsoon season set to go on until the end of August and the Indus river, swollen with floodwaters, moving steadily south, we still do not know how many Pakistanis will be affected in the end, our correspondent says.
'One in 10'
Mr Holmes pointed out that almost one in 10 of Pakistan's population had been affected by the floods and at least 6 million were in need of immediate humanitarian aid.
"The floodwaters have devastated towns and village, downed power and communications lines, washed away bridges and roads and inflicted major damage to buildings and houses," he said.
Pakistan's government had led the response to the floods by initiating rescue and relief operations and evacuating hundreds of thousands of people but could not cope on its own, Mr Holmes argued.
"While the death toll may be much lower than in some major disasters... it is clear that this disaster is one of the most challenging that any country has faced in recent years," he added.
"That is why the international humanitarian community has been asked to support the response by covering gaps where the needs inevitably exceed the government's response capacity."
Mr Holmes said he was confident the amount of aid pledged would rise as the scale of the disaster became apparent.
About $7bn was pledged within a month of the tsunami that struck Asia in December 2004.
Speaking separately, the UN humanitarian co-ordinator in Pakistan, Martin Mogwanja, called for action to stop the spread of disease.
High levels of gastroenteritis and water-borne diseases such as scabies were already being noticed, he said.
"There are literally millions of people tonight in Pakistan who are sleeping out in the open on high ground... not having adequate shelter, not even a tent or a piece of plastic sheeting," he added.
"We need to expand this response radically and very, very fast."
Enraged survivors have been physically attacking government officials in flood-hit areas, amid widespread anger at the slow pace of the relief effort.
The battle against the floods continued across all of Pakistan's provinces on Wednesday:
- In Sindh province, tens of thousands of displaced people have been flocking to Sukkur city - itself under threat from rising water levels - while the city of Hyderabad is under threat from a wave expected to hit Kotri Barrage on the Indus river in the next 24 to 48 hours
- Fresh floods in Punjab province - upstream of Guddu - caused hundreds of thousands of people to move to safer areas, in many cases for the second time in as many weeks, while Muzaffargarh, a city of 700,000 people, was evacuated
- In Balochistan province, officials say rains and floods have killed at least six people in Ziarat, Harnai and Loralai areas. Officials say that hundreds of people in these areas are trapped
Pakistan's ambassador to the UN, Abdullah Hussain Haroon, said it was difficult for people to comprehend how big a disaster it was.
"It is going to put us back so many years that we're not even starting on the infrastructure," he said.
And as so many wait for food, clean water or shelter to come their way, frustrations at the impotence of the Pakistani government grow, the BBC's Aleem Maqbool reports from Islamabad.
In terms of public opinion, the army has fared better because it has at least been visible, ferrying the stranded to safety.
But as a country that was becoming ever dependent on civilian and military aid from the outside world even before this crisis, there is no doubt it will need much more, if millions who have lost homes, livelihoods and belongings are to be spared more misery, our correspondent says.
BBC Urdu will transmit six daily bulletins in Urdu and Pashto providing vital information including how to stay safe, avoid disease and access aid. Special programmes will be broadcast each day in Urdu at 12.30, 15.30 and 18.30 and in Pashto at 12.45, 15.45 and 18.45 (local times).