With as much secrecy as the Pentagon, the United Nations has been busily counting the likely casualty toll of a war on Iraq. While the Pentagon focuses on its troops, the network of UN specialist agencies is trying to estimate what would happen to Iraqis.

The assessments are dramatic, though for reasons of internal diplomacy or because of American pressure the UN is unwilling to go public with the figures. But a newly leaked report from a special UN taskforce that summarises the assessments calculates that about 500,000 people could "require medical treatment to a greater or lesser degree as a result of direct or indirect injuries", according to the World Health Organisation.

WHO estimates that 100,000 Iraqi civilians could be wounded and another 400,000 hit by disease after the bombing of water and sewage facilities and the disruption of food supplies.

"The nutritional status of some 3.03 million people will be dire and they will require therapeutic feeding," says the UN children's fund. About four-fifths of these victims will be children under five. The rest will be pregnant and lactating women.

Although Iraq's population at 26 million is almost the same as Afghanistan's, UN agencies say the effect of war in Iraq would be far worse. Afghanistan is largely rural so that people have long traditions of coping mechanisms.

By contrast, Iraq has "a relatively urbanised population, with the state providing the basic needs of the population". Some 16 million depend on the monthly "food basket" of basic goods such as rice, sugar, flour, and cooking oil, supplied for free by the Iraqi government.

The expected bombing of Iraq's infrastructure would disrupt these supplies and the UN would struggle to send in food from outside Iraq. The electricity network "will be seriously degraded", the UN says, leaving millions without proper drinking water because treatment plants will be unable to function. At the moment 70% of the urban population has access to water from treatment plants with standby generators, but if these are also hit, the numbers at risk would escalate. Only 10% of the sewage pumping stations have generators so bombing could quickly provoke cholera and dysentery.

The United Nations high commission for refugees estimates at least 900,000 Iraqi refugees will go to Iran. No figures have been given for those who may go to Kuwait, Syria, Jordan, or Turkey. Another 2 million could be displaced inside the country.

The UN report makes no estimate of likely Iraqi war deaths. In Afghanistan it is calculated that bombing killed about 5,000 civilians directly. Up to 20,000 other Afghans died through the disruption of drought relief and the bombing's other indirect effects, according to a Guardian investigation of death rates at camps for the internally displaced. Bombing in Iraq would probably produce similar proportions of direct and indirect fatalities.

The UN estimates that city dwellers who lose their homes will be able to move to partially destroyed buildings nearby but it foresees that hundreds of thousands will escape to the countryside and be forced to sleep in the open. It says 3.6 million will need "emergency shelter".

The UN report does not make any distinction on whether the war is authorised by the security council or not, since a bomb is just as lethal whoever orders it to drop. It is taken for granted that the United States will be in charge of the targeting, and the UN will not have any influence. The report was leaked to an American non-governmental organisation and posted on the website of the UK-based anti-war group, Campaign against Sanctions in Iraq. UN officials have not challenged its authenticity. Nathaniel Hurd, who obtained it, said yesterday: "The UN may have updated some assessments but this is only likely to affect estimates of refugee flows and not the figures on damage and destruction."

Other NGOs have been conducting their own assessments. Oxfam, which has sent water specialists to the region, says half of Iraq's sewage treatment plants already do not work because of shortages of spare parts caused by sanctions. "We are particularly concerned about water and sanitation and the problems of pumping. There is no normal economy because people rely on state food distribution on a massive scale", says Barbara Stocking, Oxfam's director.

Medact, the UK affiliate of International Physicians for the Prevention of Nuclear War, estimates casualties could be five times higher than in the 1991 Gulf war. "The avowed US aim of regime change means any new conflict will be much more intense and destructive, and will involve more deadly weapons developed in the interim," it says in a report available on the first Gulf war, the UN calculated that between 3,500 and 15,000 civilians died during the war (plus between 100,000 and 120,000 Iraqi troops). A new war of the kind projected by the US could kill between 2,000 and 50,000 in Baghdad and between 1,200 and 30,000 on the southern and northern fronts in Basra, Kirkuk and Mosul. If biological and chemical weapons were used, up to 33,000 more people could die.

Medact examines detailed recent analyses by other specialists on the various tactics the US may use. The wide range of figures comes from different estimates of the degree of Iraqi resistance and the length of the war.

The leaked UN report is at www.casi.org.uk