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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.
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Smoke rising in Kyiv, which mayor Vitali Klitschko says faces a 'difficult and dangerous moment' as Russian forces step up strikes. AFP -
Pope Francis meets visitors holding the Ukrainian flag during his weekly general audience in the Vatican. AFP -
An anti-war protester demonstrates against Russia's invasion of Ukraine in front of the Nato headquarters in Brussels. AFP -
A poster in Saint Petersburg carries the letter Z, a symbol of support for the invasion, and reads: "We are proud of Russia! We are not ashamed!" AFP -
Amid the Ukraine war, there's time for a seaside stroll in Odesa. Reuters -
A child who fled from Ukraine to Belgium waits outside an immigration office in Brussels. EPA -
Ukrainian President Volodymyr Zelenskyy receives a standing ovation as he appears via videoconference to make an address to Canada's Parliament. AP -
A woman walks with a bicycle next to a building damaged during the conflict in the separatist-controlled town of Volnovakha, Donetsk region. Reuters -
A firefighter outside a destroyed apartment building in a residential area of Kyiv. AP -
Ukrainian soldiers pay tribute to Col Valeriy Gudz, who was killed in battle against Russian troops, at a cemetery in the town of Boryspil. AP -
Firefighters work to extinguish a fire in a housing block hit by shelling in the Sviatoshynsky district, in western Kyiv. AFP -
Two people were killed as a series of powerful explosions rocked residential districts in Kyiv just hours before talks between Ukraine and Russia were set to resume. AFP -
The Met Opera and Chorus performs during 'A Concert For Ukraine' in New York City. AFP -
A worker welds metal at the Interpipe Steel plant in Dnipro, Ukraine. Hundreds of Interpipe’s 10,000 employees have joined the fight against Russia. AP -
A woman is rescued by firefighters from her apartment in a burning building that was hit by artillery shells in Kyiv. AP -
Rescuers work at a building damaged by an air strike, in central Kharkiv, Ukraine. Reuters -
A woman who fled Ukraine to Belgium waits outside an immigration office, in Brussels. EPA -
Cars stuck at the Irpin River bridge, as Russia unleashes a barrage of air strikes on cities across Ukraine. AFP -
A firefighter looks at a section of a Ukrainian Tochka-U missile on a street in the separatist Donetsk region. The Russian military said 20 civilians were killed by a ballistic missile launched by the Ukrainian forces. AP -
Musician Davide Martello plays a piano near the Ukrainian border in Medyka, Poland. More than 1.76 million people have crossed the Polish-Ukrainian border into Poland, the Polish Border Guard reported on March 14. EPA -
Ukrainian President Volodymyr Zelenskyy, left, snaps a selfie with a wounded man during a visit at a military hospital after fighting in the Kyiv region.AFP -
Ukrainian soldiers carry rocket-propelled grenades and sniper rifles as they walk towards the city of Irpin, north-west of Kyiv. AFP -
Troops carry an elderly woman on a stretcher across a makeshift bridge as Ukrainians flee Irpin. AFP -
Irina Moprezova, 54, stands in front of a house that was damaged by an air strike in Irpin. AFP -
The flags of Russia and Ukraine are projected on the walls of Jerusalem's Old City, which a representative from the Jerusalem Municipality said is a show of support for diplomatic dialogue between the countries. Reuters -
A person holds a Ukrainian flag during an anti-war demonstration at the Brandenburg Gate in Berlin, Germany. Reuters -
A Ukrainian soldier takes cover as people flee Irpin. AFP -
Ukrainians shelter in an underground train station during the day in Kyiv. EPA -
A Ukrainian serviceman walks carefully with his weapon in the city of Irpin, near Kyiv. AFP -
Ukrainian people flee the city of Irpin, north-west of Kyiv. AFP -
Ukrainian servicemen carry an elderly woman on a stretcher from the city of Irpin. AFP -
A girl plays games on her tablet inside a subway carriage being used as a bomb shelter in Kyiv. AFP -
A Ukrainian firefighter drags a hose inside a large food products storage facility which was destroyed by an air strike on the outskirts of Kyiv. AP -
Ukrainian troops with a Javelin anti-tank missile on the front line in the northern Kyiv region. Reuters -
A woman carries her dog during an evacuation in Irpin. Reuters -
A Ukrainian woman takes shelter in a metro station during the day in Kyiv. EPA -
A member of the Ukrainian armed forces on patrol in Irpin. Reuters -
A Ukrainian fighter takes cover behind a car in the city of Irpin. AFP -
Firefighters at work after air strikes hit residential buildings in Chernihiv, northern Ukraine. AFP -
A man wounded in an air strike is helped by medical staff in Novoiavorivsk, western Ukraine. Getty -
The ruins of the Ukraine Hotel after recent shelling in Chernihiv. EPA
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".
