The European Union's foreign policy chief says Ukrainian pilots have started training to operate F-16 fighter jets in "several countries," including Poland.
Josep Borrell made the comments while speaking to reporters in Brussels on Tuesday.
He said that the training "will take time, but the sooner the better." He added that Western allies have opened the door for providing the aircraft.
F-16s are made in the US, but Western allies have not agreed on how many they might supply or when that might happen.
Russian leaders say supplying the jets will come with "obvious risks." They criticized the allies for becoming "increasingly involved" in the conflict.
NATO Secretary General Jens Stoltenberg said training the Ukrainian pilots does not make the alliance "party to the conflict." He added that the right of self-defense is enshrined in the UN Charter, and NATO is helping Ukraine "uphold that right."