(NEXSTAR) – Ukrainian President Volodymyr Zelenskyy has once again pleaded with U.S. lawmakers to establish a no-fly zone over Ukraine amid Russia’s attack, but it seems unlikely that the leaders in the West will reconsider.

In his virtual address to Congress on Wednesday, Zelenskyy said Russia has “turned the Ukrainian sky into a source of death” for thousands.

“Is this a lot to ask for? To create a no-fly zone over Ukraine to save people? Is this too much to ask?” he asked in his address. “A humanitarian no-fly zone, something that Russia would not be able to terrorize our free cities?”

In recent weeks, however, NATO countries had decided against implementing a no-fly zone over Ukraine, warning that such a move would provoke a much larger war in Europe. Russian President Vladimir Putin has also said that countries who align with Ukraine in the implementation of a no-fly zone would be viewed as participants in the war.

White House Press Secretary Jen Psaki, speaking to reporters on Tuesday, added that White House officials “disagree with the argument” that a no-fly zone would even be effective, despite Zelenskyy’s requests.

What then, exactly, would a no-fly zone do?

A no-fly zone would effectively mean that Ukraine’s airspace would be off-limits to any unauthorized aircraft, possibly preventing air strikes on Ukraine’s citizens and cities. This would mean, however, that Ukraine’s skies would need to be policed and patrolled by fighter planes and other aircraft to monitor for air-based threats. It’s also a possibility that these planes might be forced to shoot down any Russian aircraft that enter into Ukraine’s airspace.

“We understand [Ukraine’s] desperation, but we also believe that if we did that, we would end up with something that could end in a full-fledged war in Europe,” said NATO Secretary Jens Stoltenberg earlier this month. “We have a responsibility as NATO allies to prevent this war from escalating beyond Ukraine.”

In addition, establishing a no-fly zone over Ukraine might not be as effective as it sounds, as some of Russia’s attacks on Ukraine have been launched from within Russian airspace, using long-range missiles that can travel for thousands of miles.

Zelenskyy, likely aware that his request for a no-fly zone would not be granted, offered an “alternative” in his Tuesday address to Congress: fighter planes and air defenses.

“You know how much depends on the battlefield, on the ability to use aircraft, powerful, stronger aviation to protect our people, our freedom, our land,” he said.

It’s unclear if Zelenskyy’s words will have any bearing on the United States’ involvement in Russia’s attack on Ukraine. As of the day before, however, the White House had said it was not considering providing aircraft for Ukraine’s defense.

“Providing the planes — our military did an assessment that’s based not just on the risk, but whether it would have a huge benefit to them,” Psaki told reporters at a press briefing. “They assessed it would not because they have their own squadron of planes and because the type of military assistance that is working to fight this war effectively is the type of assistance we’re already providing.”