The president of Ukraine, Volodymyr Zelenskiy, has called on the North Atlantic Treaty Organization (NATO) and its allies to declare a no-fly zone over Ukraine in a bid to stop Russia’s air strikes against the country. But while the United States and the European Union have mobilized to provide arms to Ukrainian forces, the Western powers have ruled out declaring a no-fly zone.

In this explainer, EL PAÍS looks at what the measure involves, how Russia has responded and what’s behind NATO’s refusal.

What is a no-fly zone?

A no-fly zone is a restricted area over which certain aircraft are not permitted to fly. The measure is typically aimed at prohibiting military aircraft from carrying out strikes, monitoring or surveillance.

What advantages would it give to Ukraine?

It would prevent Russian aircraft from dropping bombs and limit the capacity of the Russian army in its invasion of a country that is more than 600,000 square kilometers.

Have no-fly zones been declared before?

Yes, on more than one occasion. For example, a no-fly zone was declared during the conflicts in Libya (2011), northern Iraq (between 1991 and 2003) and in Bosnia and Herzegovina (1993-95).

What was the reason for the no-fly zones?

In the case of Libya, the measure was established to prevent the air force of the Libyan government of Muammar Gaddafi from crushing the Arab Spring uprising in the country. In other cases, no-fly zones have been declared for humanitarian reasons or to stop the persecution and killing of certain ethnic groups within a country.

How is a no-fly zone enforced?

It is enforced militarily. Any aircraft that enters a no-fly zone can be shot down.

Who enforces it?

That depends on each case. In Libya, the no-fly zone was enforced by an international coalition. In northern Iraq, US, British and French forces were tasked with this job. And in the former Yugoslavian republic of Bosnia, NATO was in charge of the operation.

What is NATO’s position on a no-fly zone in Ukraine?

NATO has ruled out declaring a no-fly zone. “We’ve agreed that we should not have NATO planes operating over Ukrainian airspace or NATO troops on Ukrainian territory,” Jens Stoltenberg, the secretary-general of NATO, announced last Friday following an urgent meeting of the 30-member alliance in Brussels.

What are the reasons behind NATO’s refusal?

NATo has refused to establish a no-fly zone over Ukraine out of fear that it will escalate the conflict and lead to a head-on clash between Russia and the main members of NATO – the US, the United Kingdom and France – all of which are nuclear powers.

“The only way to actually implement something like a no-fly zone is to send NATO planes into Ukrainian airspace and to shoot down Russian planes, and that could lead to a full-fledged war in Europe,” Stoltenberg explained on Friday. Experts also point out that the main mission of a no-fly zone is to destroy the anti-aircraft equipment of the potential enemy, which could lead to the bombing of Russian positions in Ukraine.

According to Stoltenberg, establishing a no-fly zone “could end in a full-fledged war in Europe, involving many more countries, and causing much more human suffering.” This statement by Stoltenberg was perhaps the first time that an international leader admitted that the invasion of Ukraine could lead to a Third World War.

What has Russia said on the issue?

On Saturday, Russian President Vladimir Putin made clear that any attempt to impose a no-fly zone would be met with reprisal. “Any movement in this direction [to declare a no-fly zone] will be considered by us as participation of the respective country in an armed conflict,” he said.

The warning came after Russian Foreign Minister Sergey Lavrov accused Zelenskiy of trying to “provoke conflict” between NATO and Russia.

Putin also warned on February 24, when launching the offensive against Ukraine, that he would use all military means in his reach to prevent third countries from intervening in the conflict. The West interpreted this statement as a threat to use nuclear bombs. In the past few weeks, Putin and Lavrov have underscored on several occasions that Moscow is willing to use its nuclear arsenal. In one of their recent phone conversations, Putin also told French President Emmanuel Macron that the objectives of Russia’s “military operation” in Ukraine will be “achieved in any case.”

On Sunday, Russian Defense Ministry spokesman Igor Konashenkov warned Ukraine’s neighbors, such as Romania, against allowing the Ukrainian air force to use their air bases. “The use of the airfield network of these [neighboring] countries for basing Ukrainian military aviation with the subsequent use of force against Russia’s army can be regarded as the involvement of these states in an armed conflict,” he said.

If it gets to that point, who would have to authorize or decide on the no-fly zone?

There are no clearly defined international rules on adopting the measure. In some cases, it was done with open approval by the United Nations. One example of this was Libya: the UN Security Council voted in favor of a no-fly zone (there were 10 affirmative votes and five abstentions from Germany, Russia, China, India and Brazil). In Bosnia-Herzegovina, the UN also made a clear call for a no-fly zone to end the humanitarian disaster on one of the fronts of the Balkans conflict. In Iraq, on the other hand, the starting point were UN resolutions in favor of intervention for humanitarian reasons, but those documents did not include an explicit authorization to use military resources.

Would it be a costly operation?

Without a doubt. Ukraine is the largest country in Europe after Russia, so the area to be covered would be huge. The no-fly zone would require very frequent patrolling, and the final amount of the bill would depend on the duration of the conflict. Experts agree that Ukrainians’ resistance could lead to a long war, and note that US participation in the no-fly zone in southern Iraq cost around €700 million a year, according to a report by the Congressional Research Service.