The two no-fly zones over Iraq were imposed by the US, Britain and France after the Gulf War, in what was described as a humanitarian effort to protect Shi'a Muslims in the south and Kurds in the north.
The justification was that an acute humanitarian crisis made it necessary to infringe the sovereignty of Iraq in this way.
British Jaguar aircraft on patrol
However, unlike the military campaign to expel Iraqi forces from Kuwait, the no-fly zones were not authorised by the UN and they are not specifically sanctioned by any Security Council resolution.
BBC diplomatic correspondent Barnaby Mason says the Western powers - led by President George Bush senior - argued that their action was consistent with Security Council Resolution 688 adopted on 5 April 1991.
The resolution condemned the repression of the Iraqi civilian population and demanded that Iraq end it immediately.
It said the repression amounted to a threat to international peace and security - a phrase our correspondent says is often used to justify intervention.
But critics of the no-fly zones point out that the resolution did not say the Security Council was acting under Chapter VII of the UN Charter, which provides for enforcement action.
Clearing the wreckage from a raid in 2000
Nor did it say that all necessary means could be used.
Critics add that whatever was justified in 1991 is not necessarily justified more than 10 years later, when the reasons for continuing the air patrols may have changed.
France no longer takes part in policing the no-fly zones, and the US and the UK are now alone in the Security Council in insisting that their frequent bombing of Iraqi targets is covered by international law.
Many UK ministers say that under international law, there is a right to intervene to prevent a humanitarian catastrophe.
They point out that Iraqi leader Saddam Hussein has hurt his people before - when he used chemical weapons to kill 5,000 Kurdish villagers in the 1980s.
But French Foreign Minister Hubert Vedrine has called on Washington to redefine its policy on Iraq and criticised the recent US-British airstrikes on Baghdad as having no legal basis in international law.
"We have believed for a long time that there is no basis in international law for this type of bombing," Mr Vedrine has said.
Other countries, notably China and Russia, have condemned the no-fly zones as a violation of Iraqi sovereignty, and they insist there is no backing for the policy under international law or UN resolutions.
The northern no-fly zone was declared after the end of the Gulf War in March 1991 to protect Kurds against military action which had driven huge numbers of people across the borders into Turkey and Iran.
Subsequently, the US, UK and France set up safe havens on the ground in northern Iraq, to which the refugees returned.
In a separate move, Iraqi aircraft were also prohibited from flying over the southern half of the country, in order to hamper President Hussein's operations against the Shi'a population there.
Since UN weapons inspectors withdrew from Iraq shortly before a three-day US-UK bombardment in late 1998 known as Operation Desert Fox, the two Western powers have kept up their attacks whenever Iraqi air defences have locked onto aircraft patrolling the no-fly zones.
Baghdad says more than 300 civilians have died in these attacks, with the some of the most serious incidents being:
20 January: Six killed in raid in southern Iraq 2000
6 April: 14 civilians killed and 19 wounded 1999
28 July: Eight killed and 26 injured in northern Iraq
18 July: 14 civilians killed in raid on southern Iraq
13 May: 12 killed when planes hit residential area in the north of the country
28 February: Oil exports cut after attack damages pipeline in Mosul
25 January: About 20 dead in attacks on Basorah region
The US and British air forces have disputed some of these figures, and insist they never target civilian areas.
However, the raids have provided ammunition for Iraqi efforts to garner support for an end to its international isolation.