Ukrainian leaders and civilians have been imploring Britain and its western allies to implement a no-fly zone in a bid to stem the Russian invasion of their country.
But British prime minister Boris Johnson, US president Joe Biden and other Nato leaders have been adamant that they cannot actively get involved in the fighting.
Here is a look at some of the reasons the West is against policing a no-fly zone...
It could lead to the Third World War
Mr Johnson made clear during his trip to Estonia on Tuesday that having British service personnel enforcing a no-fly zone would be likely to mean “shooting down Russian planes”.
He said such a move would be an escalation of the conflict, and effectively mean the UK fighting Russian forces in Ukraine.
The Nato alliance, including the UK, possesses nuclear weapons, as does Russia, meaning involvement by its members could lead to nuclear warfare, with devastating consequences.
General Sir Adrian Bradshaw, Nato’s former deputy supreme allied commander, said Nato entering the struggle would “amount to 30 countries against Russia”.
“This is the Third World War in anybody’s language. We cannot afford to let that happen,” he told BBC Radio 4’s Today programme.
Ukrainian planes would not be able to take off either
The restrictions on planes taking to the air would also have to apply to Kyiv’s aircraft, a policy that could help Russian forces on the ground, Britain's defence secretary said.
Ben Wallace told Sky News that without Ukrainian planes monitoring activity from the sky, the Russian army would be “able to drive around with impunity”.
Much of the damage inflicted so far has come from land-based weapons
There is a question about how much difference a no-fly zone would make to the destruction being wrought by Moscow on the Ukrainian people.
Many of the rockets and missiles being fired on cities such as Kharkiv are being sent from ground weapons, rather than from the air.
However, that could change as Russian troops advance and, as western intelligence suggests, start to besiege urban areas.
Britain is already supplying anti-aircraft weapons to Ukraine
The British defence secretary confirmed – thought to be for the first time – that the UK has given anti-aircraft weapons to counter Russian jets.
Mr Wallace said the arms were working to frustrate the Russian air force and preventing daytime bombing raids.
He told BBC Radio 4’s Today programme: “The reason they are doing the bombing at night is… because of those weapon systems – the Russians cannot fly much in the day.”
Britain's Ministry of Defence previously confirmed the UK was supplying anti-tank weapons to the Ukrainians, but officials had been reluctant to publicly state what other arms have been supplied.
Ukrainians fighters are not trained to fly Nato warplanes
Away from calls for a no-fly zone, there has been separate pressure for the West to send Kyiv warplanes to use against Moscow.
However, Ukrainian pilots are not trained to fly the majority of Nato planes.
With the defensive alliance not willing to send armed forces into the country, it would be difficult to provide training to allow Ukrainian pilots to learn how to operate such aircraft.
The European Union had, according to Ukrainian officials, been looking to provide as many as 70 fighter planes from the Soviet-era that Kyiv pilots are trained on.
But that plan appeared to have broken down, with Politico reporting that Poland, Bulgaria and Slovakia – from where the MiG-29s were meant to come from – were against the idea, for security and supply reasons.
