The US has pledged its full support for flood-hit Pakistan, as an emergency UN session intended to boost the international response gets under way.
The US special envoy to the region, Richard Holbrooke, told the BBC that the US was doing more than any other nation to help Pakistan.
More than four million people have no shelter, and millions more need immediate assistance, the UN says.
Weeks of heavy rain have submerged large parts of the country.
There are fears of more flooding as water continues to surge south down the Indus River.
Speaking in New York, Mr Holbrooke said the US had been the first country to offer help to Pakistan in the early days of the crisis.
He said much-needed helicopters had been diverted from the war in Afghanistan to deliver aid and rescue stranded people in north-west Pakistan, where the flooding began.
"We're going to help as long as we can, as long as we're needed. We have been Pakistan's best friend in this crisis.... We are really going all out."
He said he hoped the special session of the UN general assembly would motivate countries to make a greater effort to help Pakistan.
Sense of urgency
The UN says it has raised nearly half the $460m (£295m) wanted for initial relief but the response remains slow.
The Asia Development Bank said it had offered to loan Pakistan $2bn to help it recover, while the World Bank has agreed to lend $900m for long-term reconstruction.
Speaking after visiting Pakistan last week, UN Secretary General Ban Ki-moon said the situation was "heart-wrenching".
He said he had never seen a disaster on such a scale.
UN member states are expected to adopt a resolution urging the international community "to extend full support and assistance" to Pakistan in its efforts "to mitigate the adverse impacts of the floods and to meet the medium and long-term rehabilitation and reconstruction needs".
The BBC's Kim Ghattas at the UN says the resolution will not produce any concrete action plan but is a sign of how nervous the US and the UN are about the level of international assistance given to Pakistan so far.
US Secretary of State Hillary Clinton is expected to announce an increase in US donations, said White House officials.
The session is a clear attempt to build a sense of urgency about a natural disaster that will have a lasting impact on a country that is key to the fight against the Taliban and al-Qaeda, says our correspondent.
Pakistan's leaders have warned that a failure to help the country in its hour of need could allow extremists an opportunity to create unrest.
"All these catastrophes give strength to forces who do not want a state structure," President Asif Ali Zardari said at a joint news conference with US Senator John Kerry.
Foreign Minister Shah Mehmood Qureshi, in New York for the UN meeting, also said a shortfall of aid could lead to instability.
"If things spin out of control they will contribute to political instability, and that is the last thing that we need," he told the BBC.
"That is the last thing that we need for economic growth, and that it is the last thing that we need vis-a-vis our fight against extremism and militancy."
Mr Holbrooke said the US was "not oblivious" of that scenario, but was focused on the immediate need for aid.
"As for the political and strategic implications of the situation, because Pakistan is so critical to the world in so many ways, we are totally aware of those.
"At this point, all we are thinking about... is the emergency period. After that, we will have to do an assessment of the reconstruction needs," he said.
Some 4.6 million people were without shelter, UN spokesman Maurizio Giuliano said on Thursday, most of them in the provinces of Punjab and Sindh.
Some of them would be able to return to homes they were forced to leave, he said, but many would have to rebuild houses destroyed by the force of the floods.
Pakistani authorities say as many as 20 million people are affected by the floods. Tens of thousands of villages remain underwater.
Officials and aid agencies in Pakistan say there are signs that the crisis is still deepening, as new flood waters continue to surge south along the Indus river and more flood defences collapse, forcing people to flee their homes.
There are growing health concerns for those surviving without proper shelter, food or clean drinking water.