In a private video call with American lawmakers over the weekend, Ukrainian President Volodymyr Zelenskyy made a “desperate” plea to the United States to help Kyiv get more warplanes to fight Russia’s invasion and retain control of its airspace.

U.S. Secretary of State Antony Blinken says Washington has given a “green light” to the idea and is currently “very, very actively” looking at a proposal under which Ukraine’s neighbour Poland would supply Kyiv with Soviet-era fighters and in turn receive American F-16s to make up for their loss.

However, the proposition is fraught with uncertainty and Poland has been less than enthusiastic about it in public, largely because Russia has warned that supporting Ukraine’s air force would be seen in Moscow as participating in the conflict and open up suppliers to possible retaliation. Official comment from NATO and European Union member Poland has been only to confirm continuing talks on the subject.