Ukrainian President Volodymyr Zelenskyy has pleaded with Western nations to impose a no-fly zone (NFZ) in the airspace above his country to counter Russian airstrikes but what exactly is an NFZ and what might be the consequences of enforcing such a sanction?
Zelenskyy believes that if NATO countries impose a no-fly zone over Ukraine, the country "will defeat the aggressor with much less blood".
However, Defence Secretary Ben Wallace has said that enforcing a no-fly zone "would lead to a war against Russia across the whole of Europe".
What is a no-fly zone?
A no-fly zone, also known as an air-exclusion zone, can be imposed to prevent aircraft from attacking or carrying out surveillance in a particular area of airspace.
It refers to any region of airspace where it has been decided among international allies that certain aircraft cannot fly, most notably military aircraft in a conflict.
No-fly zones are authorised under Chapter 42 of the UN Charter which states: "Should the Security Council consider that measures provided for in Article 41 would be inadequate or have proved to be inadequate, it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security.
"Such action may include demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations."
So, if non-military actions such imposing fresh sanctions, as set out in Chapter 41 of the charter, or sending equipment to Ukraine's armed forces failed to work, Chapter 42 sets out an option that would allow for a no-fly zone to be put into effect.
Air exclusion zones could also be set up for civilian purposes like protecting sensitive areas of land or protecting large sporting events from terror attacks.
How can a no-fly zone be enforced?
Defence Secretary Ben Wallace revealed, while speaking with Sky News, that a no-fly zone "would involve British fighter jets shooting down Russian fighter jets, probably over Ukraine, that would lead to Article Five triggering of NATO."
Article Five states that an armed attack against one or more of the nations in Europe or North America "shall be considered an attack against them all."
If a no-fly zone was imposed over Ukraine, NATO forces would in effect have the authority to shoot down unauthorised military aircraft in that airspace.
However, as Mr Wallace added, such a no-fly zone would also apply to Ukrainian aircraft "meaning they could not target Russian forces from the air."
Could a no-fly zone start a war?
General Sir Adrian Bradshaw, a former NATO deputy supreme allied commander, told BBC Radio 4's Today programme: "This is not like Iraq; we're up against a sophisticated enemy with very capable air defence assets."
He explained very clearly that the 30 countries of NATO going against Russia and enforcing a no-fly zone is "the third world war in anybody's language."
When asked how the West should respond to desperate pleas from Ukrainians calling for a no-fly zone to prevent genocide he responded: "Nobody can hear those cries without feeling the pain of the Ukrainians, but the reality is, that we cannot afford to bring on the Third World War."
What does Boris Johnson say about no-fly zones?
In a now-viral video, Ukrainian journalist Daria Kaleniuk was moved to tears at a press conference in Poland as she tried to explain to Prime Minister Boris Johnson why Ukrainian women and children, who are fleeing to the Ukraine border to escape being bombed, are asking for a no-fly zone. She said: "You are not coming to Kyiv ... because you are afraid because NATO is not willing to defend.
"Because NATO is afraid of World War Three but it has already started and it is Ukrainian children who are there taking the hit."
Mr Johnson responded to Kaleniuk by saying: "I'm acutely conscious that there is not enough that we can do as the UK government to help in the way that you want and I've got to be honest about that.
"When you talk about the no-fly zone ... unfortunately the implication of that is that the UK will be engaged in shooting down Russian planes and will be engaged in direct combat with Russia and that's not something that we can do or that we've envisaged."
When and where have no-fly zones been used before?
No-fly zones have been used three times for military purposes.
The first instance was in 1991 when the United States and NATO implemented no-fly zones in Iraq after the Gulf War. The aim was to prevent attacks by Saddam Hussein.
A year later, in 1992, all military flights were banned in the airspace over Bosnia. Between October 1992 and April 1993, there were more than 500 violations of the no-fly zone, but no action was taken against those who flew through the banned airspace.
Less than 20 years later, the United Nations Security Council approved a no-fly zone over Libya in 2011 to stop attacks on civilians by Gaddafi forces.
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