BBC Defence correspondent Nick Childs says the zones are now crucial to the US and British containment policy against Saddam Hussein.

"In the light of the fact that there are no inspections on the ground at the moment one could argue that the no-fly zones have become to some extent a more important plank in that containment policy at least for the time being," he said.

British Jaguar aircraft on patrol

Daily patrols are made in the no-fly zones either by British or US aircraft. At present Britain has four Jaguar aircraft based at Incirlik in southern Turkey patrolling the northern zone, and 18 Tornadoes flying over the southern zone, 12 based in Kuwait and six in Saudi Arabia.

The United States has more than 200 aircraft based in the Gulf, including B52 bombers and B1 stealth bombers, some based on the UK's Indian Ocean island of Diego Garcia.

THE ZONES

The two no-fly zones, one in the north and another in the south of Iraq, were unilaterally created by the US, Britain and France soon after the 1991 Gulf War. Iraq was banned from using all aircraft, including helicopters, in the air exclusion zones.

The Northern Zone
The Northern Zone was established in April 1991 after Baghdad mobilised helicopter gunships to quell a Kurdish uprising. Known as Operation Provide Comfort, it lies to the north of the 36th parallel covering an area of 19, 000 square miles. France pulled out of patrolling the operation in December 1996 because, it said, changes in the mission had eliminated its humanitarian aspects.

The Southern Zone
The Southern Zone was imposed south of the 32nd parallel in August 1992 to protect Shi'ite Muslims who also rebelled against Baghdad.

Known as Operation Southern Watch, it came about after the UN passed a resolution condemning President Saddam Hussein for the repression of Iraq's civilian population. At first the zone covered mainly marshlands and included Basra, Iraq's second largest city.

Late in 1996, it was extended northward, closer to the Iraqi capital, to the 33rd parallel, covering the southern third of Iraq. The expansion came after Iraq intervened in fighting between Kurdish factions around the city of Irbil, a UN-designated safe haven.

France refused to participate in patrolling the extended zone.

LEGITIMACY OF THE BAN

Baghdad has questioned the international legitimacy of the no-fly zones claiming that there are no UN Security Council resolutions mandating their enforcement.

Immediately after the southern expansion, Iraqi President Saddam Hussein said he would no longer recognise the no-fly zones and urged his troops to shoot down US planes.

US F18 returns from routine check

Nick Childs said: "Certainly I think one of the reasons why Iraq has for a long time complained that this is an infringement of its sovereignty is because to some extent the legal basis for this is ambiguous".

Washington has long argued that Resolution 688, which stipulates that Iraq cannot hurt its own people, provides the legal basis for the zones.

However US officials are aware that this position is not universally acknowledged. Russia, for instance, has stated that the zones have not been backed by specific UN resolutions.

BREACHING THE BAN

The British Ministry of Defence maintains that the no-fly zones remain exclusive.

It says no Iraqi aircraft, be they military or civilian, are allowed to fly in the zone and any other country approaching the airspace is warned off.

But the zones have been breached by Iraq on several past occasions, with varying consequences.

• December 1992 - a combat aircraft shot down an Iraqi fighter which had entered one of the exclusion zones.
• 13 January 1993 - Western forces attacked targets in southern Iraq following Iraqi military activity in the exclusion zone. Further attacks took place in late January 1993.
• July/August 1993 - US planes strike against Iraqi anti-aircraft missile sites in a series of attacks in the no fly zones in southern and northern Iraq.
• August/September 1996 - Iraqi armed forces deployed in the Kurdish safe haven in northern Iraq. The US Launches missile attacks on targets in southern Iraq and extends the no-fly zone.
• April 1997 - During the Muslim pilgrimage to Saudi Arabia, known as the Hajj, Saddam flies pilgrims both to and from Mecca. US refrains from action saying: "We're not prepared to stop what seem to be small-scale and reasonable humanitarian actions." The pilgrimage is due to begin again in early 1999.
• June 31 1998 - US F-16 fires a missile at a radar site in southern Iraq after Iraqi radar locked onto Allied jets flying a routine mission over the Iraqi no-fly zone.