Aid agencies are preparing for what some believe will become the largest humanitarian operation in history.
The UN's World Food Programme has said it fears it will be making its biggest ever appeal for cash as it addresses Iraq's humanitarian needs - more than $1bn to help feed Iraqis for about six months.
In the UK, aid agencies have launched emergency appeals and are waiting for the all-clear to enter Iraq as regions are declared safe.
UN camps are being prepared in Jordan
The humanitarian effort in and around Iraq is led by UN agencies.
Refugee camps are being set up by the UNHCR in Syria, Jordan and Iran. Aid agencies will then register with the UNHCR to run the camps.
The agency is hoping for $60m to address the crisis.
Unicef, which says it needs $18m, is the lead agency on water and sanitation and has 200 Iraqi staff in the country's main cities.
Mobile sanitation units can provide clean water for 600,000 people.
The World Food Programme has 126 staff in Iraq and has stockpiled enough food for two million people for a month.
The World Health Organisation has almost 300 staff in Iraq.
UK military aid provided by the UK government and including food, water and blankets, will arrive through the port of Umm Qasr.
The US military aid operation is based in Kuwait.
About 16 million people out of a population of 23 million in Iraq depended on food aid even before the outbreak of war.
A quarter of children under five are malnourished, while 30% of babies are born at a low birth weight.
A quarter of Iraq's population had no access to clean drinking water before the conflict - and there are fears bombing by coalition forces will reduce supplies further.
In the southern city of Basra in particular there are already water shortages.
UK Government aid
Armed forces humanitarian operation: £30m
Red Cross/Red Crescent: £32.5m
Aid agencies: £5m
Further £120m provided to DFID
And aid agencies fear that there will be outbreaks of typhoid and cholera as citizens drink polluted water from rivers.
Preparations are also underway for Iraqis fleeing their homes.
Aid agencies say that so far, most refugees are those who have left their homes in cities to stay with extended families in rural areas of northern Iraq.
But they also warn that thousands more could arrive at the borders weeks after the outbreak of war after travelling from central regions of the country.
The UN fears about 600,000 people could seek to leave Iraq, with half heading for Iran.
The other half may go to Turkey, northern Iraq, Syria and Jordan.
Meanwhile, the UK government has made £30m available to the armed forces for the delivery of immediate humanitarian relief.
It has provided £13m to the UN, and £32.5m to the Red Cross and the Red Crescent.
The international development department (DFID) is making £5m available to aid agencies.
Chancellor Gordon Brown has also announced a further £120m for the international development department.
So how are the UK agencies and other main international agencies preparing for the humanitarian effort?
SAVE THE CHILDREN
Save the Children has 60 Kurdish staff working in the towns of Irbil, Sulaymaniyah, Diyana and Dahuk in northern Iraq.
They are distributing tents, blankets, and children's clothes having stockpiled provisions ahead of the conflict.
More than 1,300 families are sheltering in schools in Diyana.
So far, refugees are not heading for the borders
The agency says most refugees are heading to the countryside from Iraq's cities and are staying with family.
There is also evidence of refugees heading north from the central and southern parts of the country.
Save the Children, which has offices in Turkey, Kuwait and Jordan, hopes to be able to send international staff into northern Iraq as soon as possible.
Cafod (Catholic Agency for Overseas Development) is part of Caritas Internationalis, the worldwide network of Catholic aid agencies.
Caritas Iraq has 14 centres in the country. Originally Well Baby Centres, they have been transformed into emergency centres to administer first aid.
There are eight centres in Baghdad, three in Mosul, one in Kirkuk, one in Basra and one in Karakosh.
The organisation has also set up emergency centres for refugees in churches around Iraq: 55 in Baghdad, 20 in Mosul, six in Kirkuk and six in Basra.
They aim to provide for 45,000 people, providing water tanks, medication, blankets, mattresses and food.
Two Cafod staff, including an engineer, are in Kuwait City and will enter Iraq when it is secure.
Another team, including a health/nutritional expert, are on standby in Jordan.
RED CROSS AND RED CRESCENT
Red Cross and Red Crescent staff are in Basra attempting to restore water supplies.
There are 10 international staff from the two organisations in Iraq along with 100 local staff.
The Red Crescent has 3,000 volunteers in Iraq and has set up 14 24-hour first aid posts in Baghdad.
Food and other supplies have been stockpiled to help 150,000 people. Blankets and medical supplies are being delivered to hospitals in Baghdad.
The basic medical needs of 180,000 people can also be addressed by the organisations.
Red Cross and Red Crescent workers are also delivering relief to families in northern Iraq.
A two-member team from Birmingham-based Islamic Relief is in Baghdad.
The organisation is planning to provide food, shelter, basic hygiene items and secure access to water initially for up to 10,000 displaced people in Iraq.
It has allocated about £1.2m for emergency relief in Iraq, with the money to be spent on blankets, tents and water purifiers, and launched an appeal for £6m.
Millions of Iraqis already rely on food aid
It also has staff in Jordan and Syria working with other agencies preparing for an influx of refugees from Iraq.
Islamic Relief has been working in Iraq since 1996, providing relief through annual food distributions and health care programmes.
Christian Aid funds and supports Iraqi partner organisations.
It is part-funding water pumps and purification units which are on standby to go into the southern city of Basra once it is safe.
In Baghdad, it is funding shelters in 50 churches and mosques, each with a newly-dug well and water pumps.
Christian Aid also has partners in the Kurdish region of northern Iraq who are working in 40 villages with refugees who have fled from cities to stay with family in rural villages.
Using Christian Aid funds, these projects can cope with 10,000 people, with refugees provided with basic provisions such as kerosene for cooking.
Aid agencies will go in to Iraq when areas are secure
The agency estimates that the food situation in the village is adequate for about two weeks.
A mobile clinic is also being funded by Christian Aid in northern Iraq.
The agency has also paid for the training of 250 search and rescue and first aid volunteers. It has also worked on training for local people on coping with chemical attacks.
Oxfam has about a dozen staff working at UN camps in Jordan on water and sanitation infrastructure.
It also has staff working at a UN camp in Syria, while four staff are planning to work at another camp in Iran once agreement for the site can be agreed with the Iranian government.
Oxfam also plans to send assessment teams into Iraq as soon as possible under the umbrella of the United Nations.
Care, one of the leading international agencies, has 30 staff in Baghdad.
It says it has plans to truck 500,000 litres of potable water per day to hospitals, health centres and other institutions in critical need in Iraq.
It says it has 72,000 litres of emergency fuel in Baghdad for this purpose.
It is also planning to use a mobile repair workshop to make emergency repairs to essential water installations, generators, disposal systems, pumps and chlorinators.
The agency says it will distribute 55 tonnes of lactose-free milk to 80 paediatric hospitals and hospitals with paediatric wings.
It is also trucking 15 tonnes of soap, washing powder and cleaning material to UNICEF supported nutrition wards in 68 paediatric hospitals in south and central Iraq.
The agency has also set up a base in Jordan and is working with the UN on a camp for 10,000 refugees.
MEDECINS SANS FRONTIERES
A six-strong team from Medecins Sans Frontieres is in Baghdad.
They include a surgeon, anaesthetist and an emergency doctor.
MSF is also sending 10 tonnes of medical supplies to Baghdad, including materials for 300 surgeries, post-operative medicines, water and sanitation supplies and some nutritional materials.
HelpAge International, a sister charity of Help the Aged which receives funding from the Department for International Development, has worked in northern Iraq since 1997.
It has stockpiled locally-purchased supplies to support older people and their families in the area.
Staff plan to travel to Baghdad when possible.
The organisation works in Amadiay area of Dohuk governorate and in the Kalar area of New Kirkuk governorate.
Many people have already left their homes, say field workers.
The agency says most of the population of towns close to Iraqi military lines in the north, such as Chamchamal and Shorish, have fled.
HelpAge International's staff in Sulaymaniyah say most have moved north to outlying towns and villages close to the Iranian border.
There are also reports that more than two thirds of the population of Dohuk has moved to villages and smaller towns, says the agency.
Partners of Tearfund in Jordan have started distributing food to refugees at a camp in Jordan.
Another of the organisation's partners is planning to feed 3,000 refugees in a church and make its three medical clinics available to Iraqi patients.
In Baghdad, Tearfund partners are distributing food and medical aid to 1,000 of the city's poorest families.
The plans are part of a long-term programme aimed at supplying water filters and food to Baghdad residents.
INTERNATIONAL MEDICAL CORPS
The International Medical Corps, based in the US, receives some funding from the UK Government.
It has staff in Turkey, Jordan and Kuwait, and plans to provide emergency surgical services, first aid, water and sanitation projects, shelter projects and general food distribution.
It plans to send in rapid response and emergency medical teams, including surgeons, nurses, water and sanitation engineers, nutritionists, food specialists and midwives.
INTERNATIONAL RESCUE COMMITTEE
The International Rescue Committee has five emergency response teams in Iraq and bordering countries.
They include specialists in water and sanitation, primary health care, logistics and child protection.
The IRC will seek to supply potable water, construct latrines, provide basic health care, distribute emergency supplies and care for vulnerable children.
Their operation is based in Amman, Jordan, while staff are also in Turkey, Kuwait and Iran.
The organisation also plans to provide further services after the conflict.
This will include health care training, health education and support to health centres, the tracing and reunification of separated families, teacher training and educational support and specialised care and protection services for traumatised children.
IRC staff will also work on repairs to and construction of water and sanitation systems, homes, schools and health facilities.
GOAL is to provide assistance and primary health care to southern Iraq.
The BBC World Service Trust is planning an emergency radio programme.
War Child plans to set up an emergency bakeries programme.
MedAir intend to provide assistance to internally displaced people in northern Iraq.