More than two dozen foreign policy experts have called for the United States and NATO to impose a partial no-fly zone over Ukraine, escalating the conflict with Russia.

The 27 officials “[urged] the Biden administration, together with NATO allies, to impose a limited No-Fly Zone over Ukraine, beginning with protection for humanitarian corridors that were agreed upon in talks between Russian and Ukrainian officials on Thursday,” according to a letter first reported by Politico on Tuesday morning.

“NATO leaders should convey to Russian officials that they do not seek direct confrontation with Russian forces, but they must also make it clear that Russian attacks on civilian areas will not be tolerated,” they added. While a partial no-fly zone for humanitarian reasons may sound innocuous, it risks leading to the same conclusion that caused top officials in the United States and Europe to reject a broader action when Ukrainian President Volodymyr Zelensky made the request in the early days of the invasion: shooting down Russian planes and sparking a direct war between the United States and a country with an estimated 6,000 nuclear weapons. In recent decades, the United States and its allies have imposed numerous no-fly zones over countries with far less powerful militaries, such as Bosnia and Saddam Hussein’s Iraq in the 1990s, and Libya in 2011.

“President Biden and NATO Secretary General [Jens] Stoltenberg have stated that neither the US nor NATO will engage Russian forces on the ground in Ukraine,” the letter continued. “What we seek is the deployment of American and NATO aircraft not to engage Russia in combat, but to avert and deter Russian bombardment that would result in a massive loss of Ukrainian lives.”

At a Feb. 28 press briefing, White House press secretary Jen Psaki stated that enforcing one would necessitate “deploying US military to enforce, which would be… potentially a direct conflict, and potentially a war with Russia, which is something we do not intend to be a part of.” On Thursday, she reiterated her stance.

“The reason why that has not been a step the president has been willing to take or we have been interested in taking is because a no-fly zone requires implementation,” Psaki told reporters. “It would require, essentially, the U.S. military shooting down Russian planes and prompting a potential direct war with Russia, the exact step that we want to avoid.”

In an interview on Sunday, Secretary of State Antony Blinken said the same thing.

“The president has also been very clear about one thing all along, which is that we’re not going to put the United States in direct conflict with Russia, that we’re not going to have, you know, American planes flying against Russian planes or our soldiers on the ground in Ukraine, because for everything we’re doing for Ukraine, the president also has a responsibility to not get us into a direct conflict, a direct war with Russia, a nuclear power, and risk a war that extends even beyond Ukraine.”

In Congress, two Republican senators, Sen. Roger Wicker of Mississippi and Rep. Adam Kinzinger of Illinois, have called for the implementation of a no-fly zone, while Sen. Joe Manchin, D-W.Va., said on Sunday that the option should be kept on the table.

NATO leadership denied Zelensky’s request once more on Friday.

“We are not a party to this conflict,” Stoltenberg said at an alliance meeting in Brussels. “As NATO allies, we have a responsibility to keep this war from spreading beyond Ukraine, because that would be even more dangerous, devastating, and would result in even more human suffering.”

Zelensky criticized the decision in a televised address late that evening, saying, “Today there was a NATO summit, a weak summit, a confused summit, a summit where it was clear that not everyone considers the battle for Europe’s freedom to be the No. 1 goal. Today, the leadership of the alliance gave the green light for further bombing of Ukrainian cities and villages, having refused to set up a no-fly zone.”