Ukrainian officials under relentless attack from Russian forces have pleaded with President Joe Biden and members of NATO to impose a no-fly zone over significant parts of Ukraine.
It's been used effectively by the U.S. and its allies several times in conflicts around the world, but experts said imposing a no-fly zone in Ukraine against Russian President Vladimir Putin's forces and his nuclear arsenal could lead to military conflict between the U.S. and Russia.
In a statement to the news website Axios, Ukrainian President Volodymyr Zelenskyy said Ukraine "can beat the aggressor" if Western allies "do their part."
What is a no-fly zone?
A no-fly zone bans military aircraft over a designated airspace to protect civilian populations from aerial attack during times of war. Typically, an international organization such as NATO or a group of countries not involved in the fighting will assume the responsibility of policing the airspace to prevent the entry of any combat aircraft to ensure the safety of civilians.
Howard Stoffer, a professor of international affairs at the University of New Haven and a former State Department official, told ABC News that a no-fly zone over Ukraine would likely lead to a confrontation between U.S. and Russian jet fighters.
"If someone's in the no-fly zone, you can't just chase them out, you have to shoot them down," Stoffer said.
The United States has been involved in several conflicts in the past 30 years where no-fly zones were implemented.
In 2011, the United Nations Security Council voted to impose one over Libya to force an immediate ceasefire of Libyan leader Muammar Gadhafi's forces. In 1991, the United States and its coalition of allies established a no-fly zone in Iraq following the Gulf War. A no-fly zone was also established by NATO in 1993 over Bosnia and Herzegovina.
But Stoffer told ABC News that establishing no-fly zones in Iraq, Libya and the Balkans is no comparison to imposing one against Russia.
"First of all, they don't have nuclear weapons," Stoffer said. "Second of all, they didn't have air superiority."
'Not a good idea’
The White House and NATO have ruled out imposing a no-fly zone in Ukraine, concluding it is not a good idea.
"It would essentially mean the U.S. military would be shooting down planes, Russian planes," White House press secretary Jen Psaki said in an interview with MSNBC on Monday.
"That is definitely escalatory ... That is not something the president wants to do. Those are all the reasons why that's not a good idea."
Instead, the United States has provided hundreds of millions of dollars in military aid to Ukraine, including anti-aircraft and anti-tank weapons. Millions more in lethal aid has been sent by NATO and European Union countries, including Germany and France.
Some American lawmakers, however, are still pressing for the U.S. to establish a no-fly zone over Ukraine.
Rep. Adam Kinzinger, R-Illinois, took to Twitter on Friday and called on the United States to declare a no-fly zone to give Ukraine a "fair fight" against the Russians.
"The fate of #Ukraine is being decided tonight, but also the fate of the west. Declare a #NoFlyZone over Ukraine at the invitation of their sovereign govt," Kinzinger tweeted.
Stoffer, who worked on the Intermediate-Range Nuclear Forces Treaty between the U.S. and the former Soviet Union, said creating a no-fly zone in Ukraine would be one of the fastest ways to trigger a war with Russia.
"The implications with war with Russia are we now have a madman in the Kremlin who is not saying things that sound like you would want to deal with. For example, he's rattling his nuclear weapons, saying they're on higher alert," he said.
Stoffer added, "While we all hate what's going on in Ukraine now, it's very hard to sit back and say we have the means to stop this but do we really have the national commitment to go to war with Russia, and I don't think we do."