Islamabad: The World Bank has promised to give $900 million (Dh3,305.78 million) to fund relief efforts for Pakistan's flood disaster, as international agencies warn millions of people are at risk from disease.

The United Nations has warned that up to 3.5 million children could be in danger of contracting deadly diseases carried through contaminated water and insects, in a crisis that has disrupted the lives of at least a tenth of Pakistan's 170 million people.

Up to 1,600 people have been killed and two million made homeless in Pakistan's worst floods in decades.

Hundreds of villages across Pakistan, one of the poorest countries in Asia, have been marooned, highways have been cut in half and thousands of homeless people have been forced to set up tarpaulin tents along the side of roads.

The World Bank says funds will come from the reprogramming of planned projects and reallocation of funds which have not yet been disbursed. How the money will be utilised has yet to be decided however.

"We are reprioritising to make the funds immediately available," said Mariam Altaf, a spokeswoman for the World Bank.

Public anger has grown as the flooding continued for the past two weeks, highlighting potential political troubles for an unpopular government as aid failed to keep pace with the rising river waters.

On Monday, some Pakistani flood victims blocked a highway to demand government help, as aid agencies warned relief was too slow to arrive for millions without clean water, food and homes.

The damage caused by the flooding and the cost of recovery could bring long-term economic pain to Pakistan and shave more than one percentage point off its economic growth, analysts say.

Pakistani stocks ended down 2.9 per cent on Monday, over fears the impact may be more damaging than first estimated.

Pakistan's High Commissioner to Britain, Wajid Shamsul Hasan, said the cost of rebuilding could be more than $10 to $15 billion. He has appealed to the international community to provide funding for relief and reconstruction for a country fighting Islamist militants, or risk potentially destabilising the whole region.

The government has been under fire for its perceived inadequate response. Islamic charities, some linked to militant groups, have stepped in to provide aid to flood victims, possibly gaining supporters at the expense of the state.

Pakistan Foreign Minister Shah Mehmood Qureshi expressed concerns over Pakistan's stability, saying it was dangerous to let the Islamists fill the vacuum.

"If a person is hungry, if a person is thirsty and you provide water, he'll not ask whether you are a moderate or an extremist," Qureshi told the British Broadcasting Corporation.

"He'll grab water from you and save himself and his children who were starved. So you have to be aware of this challenge."

Only a quarter of the $459 million aid needed for initial relief has arrived, according to the United Nations. This contrasts with the United States giving at least $1 billion in military aid last year to its regional ally to battle militants.

The UN has reported the first case of cholera. In a statement issued in New York, it said the greatest threat was from acute watery diarrhoea and dysentery, but that hepatitis A and E and typhoid fever were also significant risks.

Victims are relying mostly on the military, the most powerful institution in Pakistan, as well as foreign aid agencies for help.