The United Nations has warned of a "second wave of death" in flood-stricken Pakistan as aid agencies struggled to raise money to help the 20 million people hit by the nation's worst-ever natural disaster.

Britain branded the international response to the catastrophe "lamentable" and charities said Pakistan was suffering from an "image deficit" partly because of perceived links to terrorism.

The United Nations has launched an appeal for $US460 million ($A512.08 million), but aid groups say the response has been sluggish and flood survivors have lashed out at Pakistan's weak civilian government for failing to help.

Maurizio Giuliano, spokesman for the UN Office for the Co-ordination of Humanitarian Affairs (OCHA), said on Monday he feared Pakistan was on the brink of a "second wave of death" unless more donor funds materialised, with up to 3.5 million children at risk from water-borne diseases.

The World Bank said it had agreed to provide Islamabad with a loan of $US900 million ($A1.0 billion), warning that the impact of the disaster on the economy was expected to be "huge".

In Australia, the federal government said it would provide an additional $A24 million to support emergency humanitarian relief in Pakistan.

In a statement on Tuesday, Prime Minister Julia Gillard said the funding would be directed through the Red Cross and Red Crescent agencies and non-government organisations, and came in response to the United Nations and the Pakistan government's call for further assistance.

It brings Australia's total commitment to $A35 million.

Japan also pledged an additional $US10 million ($A11.13 million), the government said on Tuesday, which is on top of $US3 million ($A3.34 million) Japan announced on August 3.

Fresh rains have threatened further anguish for the millions affected by three weeks of flooding that has engulfed about one quarter of the stricken country, including its rich agricultural heartland.

UN chief Ban Ki-moon has urged the world to urgently speed up aid, while Pakistani Foreign Minister Shah Mehmood Qureshi said the country could not cope on its own and warned the disaster could play into the hands of insurgents.

The flooding has already cost the lives of up to 1600 people, destroyed towns and villages and major infrastructure and wiped out crops, raising deep concerns for the long term.

"We fear we're getting close to the start of seeing a second wave of death if not enough money comes through, due to water-borne diseases along with lack of clean water and food shortages," Giuliano told Agence France-Presse.

He told AFP that about six million people were at risk of deadly water-borne diseases, including 3.5 million children.

Typhoid and hepatitis A and E are also concerns, he said, adding that the World Health Organisation is preparing to assist up to 140,000 people in case of a cholera outbreak.

Cholera is endemic in Pakistan and the risk of outbreaks increases with flooding, but the government has so far confirmed no cases publicly.

One charity worker told AFP on condition of anonymity that several flood survivors had already died of the disease.

The United Nations estimates that 1600 people have died in the floods, while the government in Islamabad has confirmed 1384 deaths.

Britain's Deputy Prime Minister Nick Clegg said about one quarter of the aid had come from his country and charged that some nations had not yet grasped the scale of the catastrophe.

"The response from the international community as a whole, I have to say, has been lamentable. It's been absolutely pitiful."

Aid agencies said they are struggling to raise funds because Pakistan suffers from an "image deficit", with donors less willing to commit cash to a country so often bound up in Western minds with extremism.

Care International spokeswoman Melanie Brooks said the UN must explain to donor states that "the money is not going to go to the hands of the Taliban".

"The victims are the mothers, the farmers, children. But in the past, information linked to Pakistan has always been linked to Taliban and terrorism," she said.

The floods have sparked rage against the government in the nuclear-armed country on the frontline of the US-led fight against al-Qaeda, where the military is locked in battle with homegrown Taliban in the northwest.

"At that very crucial time this natural disaster has affected the ability and the capacity and the economy of Pakistan," Qureshi told the BBC.

"The damage and the magnitude is too large for natural resources to cope with it ... Pakistan needs your help."

Intermittent rain fell Monday, turning refugee camps into mud, keeping alive fears of further breaches in the Indus river and canals and hampering relief efforts, officials said.