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Ukraine President Volodymyr Zelenskyy has again appealed for Nato to impose a no-fly zone over his country.
Mr Zelenskyy told Nato it would protect not only Ukraine, but countries of the Atlantic alliance from Russian air attacks.
US and Nato allies have so far refused — with UK Defence Secretary Ben Wallace arguing it would strip the Ukrainian armed forces of their ability to strike Russian targets from the air.
Many say any attempt to establish a no-fly zone would place Nato forces in direct conflict with nuclear-armed Russia and could spark a wider, even more dangerous conflict.
Here is what you need to know about no-fly zones, or NFZs.
What is a no-fly zone?
It is a prohibition on all or certain types of aircraft flying through a designated airspace, over a country or region.
No-fly zones require one or more parties being willing to enforce them — meaning being ready and able to shoot down any aircraft violating the space.
US and Nato officials say flatly that for a Ukraine NFZ to work, their own jets would have to be prepared to shoot down Russian aircraft, effectively making them direct participants in the war.
“If we did that, we'll end up with something that could end in a full-fledged war in Europe, involving many more countries and causing much more human suffering. So that's the reason why we make this painful decision,” Nato Secretary General Jens Stoltenberg said on March 4.
A no-fly zone could allow civilians, who are becoming casualties at a staggering rate, to flee the conflict more safely.
Where have no-fly zones been put in place?
They have been used in attempts to limit an ongoing conflict on the ground and to protect certain populations. But they are costly, requiring constant air patrolling and monitoring.
From 1991 to 2003 the US, France and Britain enforced no-fly zones over Iraq after the 1991 Gulf War, to protect Shiite populations in the south and minority Kurds in the north from air attacks by the Saddam Hussein's Sunni government.
From 1993-1995 Nato enforced a UN-declared NFZ over Bosnia.
And in 2011 Nato also enforced a UN-approved NFZ over Libya during that country's civil war.
A full NFZ by Nato would mean that Russia's superior air power could not be deployed against Ukrainians — but also that the Kyiv would not be able to fly its military aircraft and attack drones against the Russians.
What are the risks of a no-fly zone over Ukraine?
Russian President Vladimir Putin said any country trying to enforce a Ukraine NFZ “will be considered by us as participation in an armed conflict by that country".
Experts have also questioned how much protection a NFZ would offer Ukraine, as Russia has launched attacks from its own airspace.
A US Department of Defence official pointed to Russia's missile attack on Sunday on a base in western Ukraine as an example. The Russians fired about 24 cruise missiles from aircraft flying over Russian territory at the time.
“A no-fly zone inside Ukraine would have had no effect on this particular set of strikes,” the official said on Monday. It “would not stop all of the air activity that is going on".
