ISLAMABAD-- Only a small fraction of the six million Pakistanis desperate for food and clean water have received any help as the United Nations battled donor fatigue and appealed urgently Tuesday for more funds. With hundreds of villages marooned and highways and bridges cut in half by swollen rivers, food rations and access to clean water have only been provided to around 500,000 million flood survivors, the UN said. 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. "We have a country which has endemic watery diarrhoea, endemic cholera, endemic upper respiratory infections and we have the conditions for much much expanded problems," UNICEF Regional Director for South Asia Daniel Toole told a news conference. "We cannot spend pledges. We cannot buy purification tablets, we cannot support Pakistan with pledges. I urge the international community to urgently change pledges into cheques." Up to 1,600 people have been killed and two million made homeless in Pakistan's worst floods in decades. The United Nations has reported the first case of cholera, but only a third of the $459 million (293 million pounds) aid needed for initial relief has arrived. "Only a limited proportion of food and water needs have been met. One of the major reasons for this is funding," U.N. spokesman Maurizio Giuliano told Reuters, adding the flood's slower unravelling compared to earthquakes and Tsunamis had dampened donor response. "Floods do not come in 30 seconds ... but the humanitarian needs are greater than in Haiti."