A no-fly-zone would spell war for any third party who tries to enforce one over Ukraine, Russian president Vladimir Putin has warned.
His chilling words come as Russia continues its invasion of neighbouring Ukraine, with shelling and bombs continuing to hit cities.
Nato has said a no-fly zone, which would bar all unauthorized aircraft from flying over Ukraine, could provoke widespread war in Europe with nuclear-armed Russia - and has resisted Ukraine's calls to enforce one.
What is a no-fly zone?
A no-fly zone, also known as a no-flight zone, or air exclusion zone, is a territory or area established by a military power over which certain aircraft are not permitted to fly.
This could include jet fighters, drones or helicopters.
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There are several articles in the UN Charter that can serve as a basis for establishing a no-fly zone, such as Chapter VII, Articles 39 and 42, in which the Security Council can identify a "threat to the peace, breach of the peace, or act of aggression" and authrorises the use of force to restore the peace.
No fly-zones have been used in the past during the Bosnian War in 1992 so the airspace could be cleared for delivery of humanitarian aid and during the first Libyan Civil War in 2011 to protect civilians from airstrikes and missiles.
It could lead to the World War Three
Speaking at a meeting with female pilots on Saturday, President Putin said Russia would view “any move in this direction” as an intervention that "will pose a threat to our service members.”
“That very second, we will view them as participants of the military conflict, and it would not matter what members they are,” the Russian president said.
ITV News Europe Editor James Mates explains why a no-fly zone is not being considered
Ukrainian President Volodymyr Zelenskyy has pushed Nato to impose a no-fly zone over his country, warning that “all the people who die from this day forward will also die because of you” as Russian forces were battering strategic locations in Ukraine.
Prime Minister Boris Johnson made clear earlier this week that having British service personnel enforcing a no-fly zone would be likely to mean “shooting down Russian planes.”
ITV News Political Editor Robert Peston said that would be an act of war by a Nato country against Russia and at that point we would genuinely be seeing the start of World War Three.
What other solutions are there to help Ukraine defend itself from the skies?
Following the most recent round of talks between US President Biden and Ukraine's President Zelenskyy, other possible means of supporting the aerial battle have been floated.
ITV News US Correspondent Emma Murphy reported "the message from the White House is that the US is doing everything it can to support as much as possible without actually going anywhere near that no-fly zone issue".
The message from Ukraine to the West is that control of the air is essential to protect citizens on the streets of bombarded cities in the country.
Already stinger anti-aircraft missiles have been supplied to Ukraine.
Another option, requested by President Zelenskyy, is that Ukraine be supplied aircraft of their own to help defend themselves after many of their machines were damaged or destroyed in the first ten days of fighting.
Emma Murphy reported there is discussion about getting Russian-made aircraft, currently in Poland, to Ukraine.