An unnamed U.S. official confirmed to the AP news agency on Wednesday that the planes had arrived, though the Ukrainian government declined to comment on the reports. It is not yet known how many planes have been sent or by which countries.

The F-16 specializes in suppressing enemy air defenses. The plane can fly at twice the speed of sound and has a range of over 3,000 kilometers. NATO members Belgium, Denmark, the Netherlands and Norway have promised to supply more than 60 of the jets though Kyiv says it would need at least 130 of the aircraft to counter Russia’s air superiority.

U.S. President Joe Biden authorized the provision of American-made combat aircraft in August 2023, since when the U.S. has been training Ukrainian pilots to fly F-16s. Bloomberg, which was the first agency to report the arrival of the planes, said it was not clear whether Ukrainian pilots were yet ready to fly them or whether more time would be needed.

Kyiv has been pushing its Western allies for months to provide F-16s but many countries were reluctant to send them for fear it would escalate the war.

Some analysts have suggested that recent mass drone attacks by Russia may indicate a surveillance operation to identify bases where F-16s may be deployed.