Ukrainian President Volodymyr Zelensky has repeatedly urged Western leaders to set up a "no-fly zone" over Ukraine, but it seems unlikely to do so in the short term.
Why it matters: The establishment of a no-fly zone (NFZ) would mark a major escalation of the war – potentially allowing NATO (Ukraine is not a member of) to engage directly in a conventional conflict with a nuclear power.
So far, the United States and other major powers have ruled out the possibility of establishing an air defense zone in Ukraine.
No-fly zone explanation
No-fly zones are airspace that certain aircraft are not allowed to enter.
In the context of conflict and war, it is often used to prevent prohibited aircraft from entering airspace to launch attacks, transport troops and weapons, and conduct surveillance.
The no-fly zone must be enforced militarily, including shooting down prohibited aircraft.
What Ukraine wants
Zelensky told Axios this week that Ukraine wants to establish a no-fly zone in "important areas" of the country.
Zelensky said: "If the West did this, Ukraine would defeat the invaders with much less blood." For now, Zelensky is still in Ukraine besieged by Vladimir Putin.
Ukraine's president has repeatedly reiterated that call and said on Thursday that if the United States and NATO do not establish a "no-fly zone," they should provide warplanes that would allow Ukraine to defend itself.
Why the West is unlikely to act
If Western countries, especially NATO, set up no-fly zones, they will be responsible for enforcing them, which could mean shooting down banned Russian military aircraft.
Howard Stoffer, a professor at the University of New Haven, said: "If NATO exerts this kind of pressure, we will shoot down even one Russian plane and we will be at war with Russia." Stouffel told Axios that this would be a major upgrade and that's not something NATO wants to do at this time.
Unlike past conflicts in which nuclear-weapon-free zones have been imposed, Russia has a strong and highly sophisticated military force and is a nuclear power.
Stouffel, who has worked at the State Department for more than 20 years, said there are still significant logistical hurdles.
NATO not only needs to decide which countries are responsible for this, but it also has to build a "very complex" defense system to oversee and enforce it.
What the West says
President Biden and the White House have repeatedly said the United States will not send troops to fight Russia in Ukraine, including enforcing a national air defense zone.
"This will require the U.S. military to shoot down a Russian aircraft and cause ... A potential direct war with Russia — that's what we want to avoid," White House press secretary Jen Psaki said Thursday.
NATO Secretary-General Stoltenberg also excluded the no-flight zone, saying on Friday: "We understand the desperation, but we also think that if we do [set up a no-fly zone], we will get some that could lead to all-out war in Europe, including [many] more countries and more suffering."
"We are not part of this conflict and it is our responsibility to ensure that the conflict does not escalate and spread beyond Ukraine," he added.
Where nfz is used
Libya: In 2011, the United Nations Security Council authorized NATO to establish a no-fly zone over Libya to "protect civilians threatened by attacks in the country."
Bosnia: From April 1993 to December 1995, NATO enforced a no-fly zone over Bosnia.
Iraq: After the 1991 Gulf War, the United States and coalition countries established two no-fly zones in Iraq.
Yes, but: the militaries of the United States and other countries that enforce non-restricted areas are far superior to the militaries they face in these cases.
Bottom line
Stouffel said it was "too early" to set up a no-fly zone.
"We don't want to get involved in a conventional conflict with Russia because it could quickly escalate to tactical and strategic nuclear levels," he added. "Then we will face the end of history as we know it."