The jet came down and its pilot died while it was approaching a Russian target, the Ukrainian General Staff said.

The F-16s had "demonstrated high efficiency" and downed four Russian cruise missiles, it added. Ukraine said Russia launched more than 200 missiles and drones that day targeting the energy sector.

"Connection with one of the aircraft was lost while it was approaching the next target. As it turned out later, the plane crashed, and the pilot died," the statement said.

A U.S. defense official told Reuters that Monday’s crash did not appear to be the result of Russian fire, and possible causes from pilot error to mechanical failure were still being investigated.

Ukraine has not given details about the size of its new fleet, though the loss left a significant dent. The Times of London has cited a source as saying Ukraine had six of the jets.

CHALLENGES

President Volodymyr Zelenskiy said on Aug. 4 that Ukraine did not have enough pilots trained to use the F-16s or enough of the jets themselves.

Ukraine’s air force western command said on Facebook that pilot Oleksiy Mes died in a combat mission on Monday.

"Oleksiy saved the Ukrainians from deadly Russian missiles. Unfortunately, at the cost of his own life," the statement said.

Mes went by the call sign Moonfish, and CNN reported in 2023 that he was training for F-16 missions.

The jets’ arrival was a milestone for Ukraine in the fight against the full-scale invasion Russia launched 2-1/2 years ago.

Military analysts have said the small number of F-16s, while significant, are unlikely to be a turning point in the conflict.

Kyiv has been urging allies to supply modern jets since the start of Russia’s invasion to bolster its small and old post-Soviet fleet.

Before Kyiv received F-16s this year, Russia had more time to prepare defences and Ukraine had to use a depleted air force that is a fraction of the size and sophistication of its enemy’s.