Ukraine’s leader President Zelensky has asked the West for a no-fly zone to be implemented as a preventative measure as his country comes under aerial attack from Russia.
He said it is not intended to drag NATO into a war with Russia, but his pleas so far have been rejected.
UK Prime Minister Boris Johnson said RAF jets could be forced to attack Russian planes if there was a no-fly zone, which would result in a much wider conflict.
Here we explain what a no-fly zone (NFZ) is, why NATO is refusing to impose one and how they have been used in the past.
What is a no-fly zone?
A no-fly-zone is an area where aircraft are barred or restricted from flying, normally for security reasons.
They are often used in war zones to stop an aggressor flying military aircraft over its own land.
It prevents attacks on civilians or military targets, as well as carrying out surveillance.
Opposing fighter jets patrol the zone in search of any aircraft breaking the rules.
Any rogue planes could be forced to land or escorted away - and they could ultimately risk being shot down.
No-fly zones can also be used over major events like the Olympics or over certain buildings.
One was imposed around the royal residence of Windsor Castle this year to permanently limit how close aircraft can get.
A no-fly zone over Ukraine would mean that NATO forces would engage directly with any Russian planes spotted in those skies and shoot at them if necessary.
Why is NATO refusing to impose one over Ukraine?
NATO countries including the UK and America have refused to impose a no-fly zone over Ukraine, to prevent a direct war from breaking out.
NATO’s secretary general Jens Stoltenberg has repeatedly ruled out a no-fly zone over Ukraine, telling NBC on 28 February: “We have no intention of moving into Ukraine, either on the ground or in the air.”
On Saturday 5 March, Mr Stoltenberg reiterated this message, arguing that such a move could result in a “full-fledged war in Europe involving many more countries”.
“We are not part of this conflict,” he added.
“We have a responsibility as NATO allies to prevent this war from escalating beyond Ukraine because that would be even more dangerous, more devastating and would cause even more human suffering.”
Fighter jets would be in direct conflict with Russian planes and could ultimately be forced to shoot one down.
This could start a direct war between NATO countries and Russia - and potentially spiral out of control.
A tearful Ukrainian journalist confronted Boris Johnson over why NATO is not implementing a no-fly zone on Tuesday 1 March at a news conference.
“Ukrainian women and Ukrainian children are in deep fear because of bombs and missiles which are coming from the sky,” Daria Kaleniuk said.
Mr Johnson aid UK jets would ultimately have to be "engaged in shooting down Russian planes".
"That is not something that we can do or that we’ve envisaged. I think the consequences of that will be truly very, very difficult to control," he said.
UK Defence Secretary Ben Wallace said on BBC Radio 4’s Today programme: “I won’t trigger a European war, but what I will do is help Ukraine fight every street with every piece of equipment we can get to them, and we will support them.”
America has also ruled out the idea.
The White House spokesperson said it could trigger "a direct conflict, and potentially a war with Russia".
What is the military view?
The Chief of the Defence Staff, Admiral Sir Tony Radakin, has said a no-fly zone over Ukraine “would not help”.
He told the BBC’s Sunday Morning programme: “The advice that we as senior military professionals are giving our politicians is to avoid doing things that are tactically ineffective and definitely to avoid doing things that tactically might lead to miscalculation or escalation.
“The no-fly zone would not help.
“Most of the shelling is coming from artillery, most of the destruction is coming from artillery, it’s not coming from Russian aircraft.
“If we were to police a no-fly zone, it means that we probably have to take out Russian defence systems and we would have Nato aircraft in the air alongside Russian aircraft, and then the potential of shooting them down and then that leads to an escalation.”
However, a former Nato supreme commander in Europe has broken with the military consensus to call for the West to enforce a no-fly zone over Ukraine.
General Philip Breedlove said the move could be carried out without “bellicose rules of engagement” that would spark a wider conflict with Russia.
General Breedlove asked: “How many casualties does it take before we take a different approach to this war?”
“I think there’s 42 million or so Ukrainians. Does it take 42 million to convince the West that we should have a different approach to this war? This is the questions that need to be going to leaders now.”
General Breedlove, a US air force general who served as Nato Supreme Allied Commander Europe between 2013 and 2016, argued a “humanitarian no-fly zone” would be possible.
It would be “one in which we go in with a decidedly non-bellicose set of rules of engagement,” he said in an interview with Times Radio.
“Those rules of engagement whereby we talk to our enemy, and we say we are not going to fire on you unless you fire on us,” he added.
How would a no-fly zone benefit Ukraine?
Russia’s air force is its principal military advantage over Ukraine, and so a no-fly zone could make a significant difference to prevent attacks.
Recent satellite pictures showed rows of attack helicopters ready to strike.
President Zelensky says it would be a preventative measure and not intended to drag NATO into a war with Russia.
However, Ukraine’s air forces are still operational with its fighter jets carrying out defensive missions.
A fleet of their Turkish-made drones equipped with laser-guided missiles has had apparent success in destroying Russian positions.
Where have no-fly zones been used before?
After the first Gulf War in 1991 the US, UK and France enforced two no-fly zones in Iraq to prevent Saddam Hussein from attacking the country’s Kurdish and Shia Muslim populations.
This was done without United Nations backing.
In the 1990s NATO countries also implemented a no-fly zone during the Bosnian War in Operation Deny Flight.
In 2011 Libya came under a no-fly zone to protect its people from attacks from the Gaddafi regime.
British RAF jets were among those involved.
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