* Warnings militants may exploit chaos
* Eight million still desperate for food, clean water
* Kerry in disbelief over flood damage (Recasts with Kerry and Zardari)
By Alistair Scrutton
ISLAMABAD, Aug 19 (Reuters) - Pakistani President Asif Ali Zardari and a senior U.S. senator warned on Thursday that Taliban insurgents are trying to exploit rising anger over the country's worst floods to promote their cause.
More than four million Pakistanis have been made homeless by nearly three weeks of floods, the United Nations said on Thursday, making the critical task of securing greater amounts of aid more urgent.
Eight million people are in urgent need of humanitarian assistance and many may not care where they get it.
The floods began washing away villages and destroying roads and bridges just after the government had made progress in stabilising the country through offensives against Taliban.
Islamist charities, some with suspected links to militant groups, stepped in to help victims, possibly boosting their image at the expense of the U.S.-backed government, which is still accused of being lax nearly three weeks into the crisis.
U.S. Senator John Kerry, who visited flood-hit areas with Zardari, said action must be taken to prevent anyone from exploiting frustrations.
"We need to address that rapidly to avoid their (Pakistani's) impatience boiling over, and people exploiting that impatience and I think it's important for all of us to understand that challenge," Kerry said, in a clear reference to the Taliban. "We also share security concerns."
For a graphic on Pakistan's floods, click
For a story on agricultural costs of floods [ID:nSGE67B050]
For an analysis of risks to watch in Pakistan, click
For a slide show, click link.reuters.com/sum54n
For more Pakistan stories, click [nAFPAK], or click
About one third of Pakistan has been hit by the floods, with waters stretching tens of kilometers from rivers.
In a small town in Punjab, people waved empty pots and pans at a military helicopter, wondering, like millions of others, when food supplies will arrive.
Aid agencies have been pushing for more funding as they try to tackle major problems such as food supplies, lack of clean water and shelter and outbreaks of disease.
The U.S. needs a stable Pakistan, which it sees as the most important ally in the war against militancy, especially in neighbouring Afghanistan, where a Taliban insurgency is raging.
In a sign of growing concerns over the ramifications of the floods, Kerry said $200 million from the $7.5 billion U.S. aid package for Pakistan over five years, which he co-authored, would be diverted to the relief effort.
The bill was unpopular in Pakistan as it ties some funds to fighting militancy, to cooperation in stopping nuclear proliferation and ensuring Pakistani civilian government dominance over the military.
Kerry said he was shocked after seeing miles of destroyed homes and displaced people in camps in sweltering heat.
Floods have ruined crops over more than 1.6 million acres, hammering the mainstay agriculture industry. Aid workers say water could stagnate on the surface for months, making planting difficult.
The government also faces the prospect of food riots and social unrest.
Zardari, who drew a hail of criticism after he left on a trip to meet the leaders of Britain and France as the disaster unfolded, also said militants could capitalise on the floods.
"There is a possibility that some, the negative forces, would exploit this situation, this time of need," he told a joint news conference with Kerry, the Democratic chairman of the Senate Foreign Relations Committee.
"They would take babies who become orphans and then put them in their own camps, train them as the terrorists of tomorrow." (Additional reporting by Faisal Aziz and Sahar Ahmed in Karachi, Zeeshan Kaider and Kamran Haider in Islamabad, Stephanie Nebehay in Geneva, Rosemarie Francisco in Manila and Jonathan Thatcher in Singapore) (Writing by Michael Georgy; Editing by Alistair Scrutton)