SLAMABAD - The world must act quickly to stop militants exploiting Pakistan's devastating floods to cause social unrest, the country's president and a senior US senator said Thursday.

In a joint news conference with President Asif Ali Zardari, John Kerry said the international community had to step in and help people whose lives have been upended "to avoid their impatience boiling over or people exploiting that impatience".

Kerry, the most senior US policymaker to visit the country since unprecedented floods have claimed 1,500 lives, was speaking as the United Nations General Assembly readied for a crisis meeting in New York.

Around 4.6 million people are still without shelter following the wave of destruction wreaked by the worst flooding in Pakistan's history, the UN said.

It estimates 20 million people have been affected and a fifth of the country is under water with the risk of cholera, typhoid and hepatitis growing.

Anger at the response to the devastation in Pakistan was palpable.

At a camp Kerry and Zardari visited in Jampur town, survivors spoke of their dissatisfaction over sluggish relief efforts, an AFP photographer said.

One young man brandished his national identity card before the president and shouted: "I got nothing."

Speaking to reporters after visiting the camp, Kerry said: "None of us wants to see this crisis provide an opportunity or an excuse for people who want to exploit the misfortune of others for political or ideological purposes.

"It is important for all of us to work over time to provide the assistance that is necessary."

The nuclear-armed nation of 167 million is a top US foreign policy priority due to concerns over Islamist extremism.

Washington says its porous border with Afghanistan provides cover for extremists to launch attacks on US-led troops fighting a nine-year insurgency.

Pakistan is also locked in battle with homegrown Taliban who have been blamed over a three-year bombing campaign that has killed more than 3,570 people.

Kerry, who visited ravaged areas in Punjab, spoke of "miles upon miles of destroyed homes, of people dislocated, of people in camps in great heat losing their possessions, growing frustrated, worried about the future.

"We need to address that, all of us rapidly, to avoid their impatience boiling over or people exploiting that impatience," he said.

Many survivors have expressed fury against the weak civilian government, blaming the authorities for failing to provide adequate relief.

"There are 20 million people dispersed, 20 million people on the roads, 20 million people hungry," Zardari acknowledged.

"We are giving them everything we've got. There is a possibility that some negative forces would exploit this situation.

"I am sending an SOS on that. All this catastrophe gives strength to those forces who do not want a state structure."

In New York, Pakistan's Foreign Minister Shah Mehmood Qureshi sought to reassure donors his corruption-tainted country could be trusted with foreign aid ahead of the General Assembly meeting called to speed up delivery of the estimated 460 million dollars needed for immediate relief.

Qureshi said he would tell the assembly session "what a serious challenge Pakistan is facing at a time when the government of Pakistan has succeeded in building a consensus against extremism and terrorism".

He pledged that his government would put in place mechanisms "that are transparent and accountable" to handle the international aid now pouring into the country.

At camps for the displaced from across the country, survivors are battling with crippling heat, miserable sanitation and swarms of mosquitoes.

Many fled their homes with just the clothes on their backs and have been forced to drink contaminated water, causing diarrhoea.

The Asian Development Bank said it would provide two billion dollars to repair roads, bridges, power lines, homes, schools, medical facilities and farm structures, and the World Bank has promised to lend 900 million dollars.

"The extent of human suffering caused by the floods cannot be easily quantified, nor can the damage wrought upon the country's physical and social infrastructure," said the ADB's chief for central and west Asia, Juan Miranda.

"But what is clear is that this disaster is like no other in living memory, and that our response must also be unprecedented, equal to the need and fast."