BBC News Online's Heather Sharp looks at the expected humanitarian fall-out in the war with Iraq, the preparations under way and the problems aid agencies face.
Aid agencies fear for ordinary Iraqis if war breaks out
Iraq's vulnerable population
Fears of a humanitarian disaster are partly based on the precarious conditions that Iraqis currently live under.
More than half the country's 23 million people are under 15, a quarter of children under five are malnourished and 30% of babies are born at a low birth weight.
A quarter of Iraq's people have no access to clean drinking water, and UNICEF fears tens of thousands of children are so weak that diarrhoea caused by dirty water could kill them.
A statistical view of daily life in Iraq
Five million people depend on a sewage system which is driven by electrical pumps.
Many of these do not have back-up generators which would keep them working if, as in the 1991 Gulf War, bombing were to knock out power supplies.
Food rations suspended
One of the biggest concerns is the suspension of the vast distribution system for monthly food rations bought under the UN's Oil-For-Food programme.
About 60% of the population - 16 million people - rely almost totally on these handouts of wheat, flour, sugar, rice, milk powder, tea, detergent, pulses and cooking oil.
Programme aims to give Iraqis 2,470 calories per day
570,000 tonnes of food a month
44,000 distribution agents
Five entry points
$1.27bn for current 6 months
The programme was suspended as UN staff were evacuated from Baghdad after the US issued its 48-hour deadline for Saddam Hussein to leave Iraq.
A new UN Security Council resolution would be needed for the programme to restart under different conditions - either in the absence of UN staff in Iraq, or, if the Iraqi Government were to fall, with another body authorised to spend Iraqi oil money on humanitarian supplies.
Even if the deal were authorised again, war would severely disrupt the infrastructure of the vast and efficient distribution network.
A widely-leaked draft UN report from the UN Office for the Co-ordination of Humanitarian Affairs (UNOCHA) dated early January said that an attack from Kuwait or southern Iraq would result in its "more or less immediate breakdown".
Another draft report expressed concerns that Umm Qasr, a key port and one of five official aid entry points, would be rendered unusable.
The White House has said "a tremendous effort" would be made to "minimise disruption" to the Oil-for-Food distribution system.
In recent months the Iraqi Government has begun giving families two months' rations at a time to allow them to stockpile supplies.
But the UN Office for the Iraq Programme said in mid-March that most households' reserves would last no more than six weeks, partly because some families were so poor they were selling their rations.
Christian Berthiaume, spokesman for the World Food Programme (WFP) says there is currently "very little food" left in the Oil-For-Food warehouses in Iraq.
The UN funding gap
International aid agencies have been implementing contingency plans based on estimated numbers of people who would flee their homes or need emergency aid during the war.
But four key UN agencies - UNHCR, UNICEF, WFP and the World Health Organisation - have only received about $44m of the $123.5m they have requested to meet their targets - and the targets themselves may be based on conservative estimates.
A crucial consideration is the length of the war. If Oil-For-Food rations are not reinstated within six weeks, millions of people could be left with very little food.
The WFP has been rushing to stockpile 32,000 tonnes of food in the region - enough to help 900,000 people for 10 weeks in the initial stages of war.
WAR BUDGET/AID BUDGET
Cost of war to US (per month): $24bn
Cost of post-war occupation to US: $1-4bn
UN agencies need: $123.5m
By early March it had received only $7.5m of the £23m it has requested, and had food stocks for only 500,000 people in place.
But it is concerned that it may have to help up to 10 million people, a mammoth task which Ms Berthiaume stresses would require huge donor support: "We are talking about hundreds of thousands of tonnes and hundreds of millions of dollars."
The UNHCR has requested $60m to provide items such as blankets and plastic sheeting for 600,000 refugees.
It has received about $21m and expects only to have supplies stockpiled for 350,000 by the end of March.
More than one million refugees fled northern Iraq in the 1991 Gulf War
Some of this aid has been redirected from existing programmes in the Balkans and Afghanistan.
Furthermore, the leaked UNOCHA draft report suggested much higher working figures for refugee numbers.
It said between 600,000 and 1.1 million people might try to flee Iraq, with up to a further 900,000 likely to be displaced internally.
The United States, Australia, UK, Norway and Canada are among the main donors so far.
Elizabeth Byrs, spokesperson for UNOCHA, told BBC News Online many donors did not want to commit funds while diplomacy was continuing, believing that this would give the impression that war was inevitable.
The aid burden of war would also be shared with a host of non-governmental organisations, including the International Committee for the Red Cross, which has 110 staff in Iraq, including 10 expatriates, who are expected to stay in the country.
US and UK plans
The US and UK governments have their own operation planned, not least because under the Geneva Convention, occupying forces are responsible for civilians in areas under their control.
Their first phase would follow military action swiftly with aid to civilians.
The US is stockpiling blankets, water containers and shelter materials for one million people - although in late February some of these were stored in "forward warehouses" in Maryland, Miami, Honduras and Guam.
Nearly three million packs of daily food rations are also being stockpiled and pre-positioned.
1991 GULF WAR
100,000 - 120,000 Iraqi military deaths
3,500 - 15,000 Iraqi civilian deaths
15,000 - 30,000 refugee deaths
More than 1 million Kurds fled northern Iraq
Source: 1991 UN report
Additionally, a 60-person team of civilian experts has been assembled to be "embedded in the military force", with troops providing security for humanitarian operations.
This team is intended to be a point of contact and information exchange for other aid agencies.
It would also work closely with the newly set up Humanitarian Operations Centre in Kuwait, which brings together the Kuwaiti authorities and the US military.
Many non-governmental agencies have, however, expressed deep concerns about working closely with US forces and receiving support from countries which are backing war.
A second key US and UK priority is therefore to secure as soon as possible a UN mandate for international and non-governmental aid agencies.
This would enable them to provide humanitarian relief without being associated directly with occupying forces.
Some charities, including Oxfam, have said they will refuse money from "belligerent countries" during any war in Iraq.
"We will not take funds that might allow a government to use a humanitarian operation as an instrument of foreign policy, thereby increasing the chances of war or prolonging it once it starts," said Oxfam International Executive Director Jeremy Hobbs.
Aid agencies and regional governments are preparing to receive hundreds of thousands of refugees.
Estimates of predicted population movements very widely, and numbers would be heavily influenced by the course and length of a war, the availability of food, water and fuel and any retaliation against civilians by Saddam Hussein's forces.
Neighbouring countries' plans:
Iran: 10 camps planned, each housing 20,000 - entry to Iran only allowed in "life-threatening" situations
Turkey: Will help on Iraqi side of border, wants to send troops into northern Iraq to prevent influx of mainly Kurdish refugees.
Jordan: Two camps being built, each reportedly expected to hold 20,000 to 30,000. The border is closed but some people may be allowed to cross "for humanitarian reasons".
Syria: UN agencies are preparing a camp for 10,000.
Saudi Arabia: The government says it has prepared for 100,000 refugees but will not allow them beyond the border region.
Kuwait: Will help refugees on Iraqi soil.
Internally displaced: Population movements have already begun, with Save the Children reporting tens of thousands of people fleeing Irbil in northern Iraq for mountainside villages.