Ukraine's Air Force says US has joined investigation into deadly F-16 crash.

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The U.S. has joined the investigation into the deadly crash of an F-16 fighter jet in Ukraine earlier in the week, Air Force Commander Mykola Oleshchuk said on Aug.

30. Ukraine's General Staff confirmed the previous day that the F-16, which had been recently delivered to the country and was being operated by pilot Oleksii Mes, had crashed while defending against a mass Russian drone and missile attack against the country on Aug.

26. Mes was killed in the crash.

Oleshchuk said that Ukraine had "received a preliminary report" from the U.S., which he said "has already joined the investigation into the causes (of the crash)." The previous day, Ukraine's Defense Ministry said it had established a special commission to look into what caused the crash. At the same time, Pentagon spokesperson Sabrina Singh said on Aug.

29 that she was "not aware of any assistance or requests for assistance from the Ukrainian side to us about this particular incident." Singh declined to go into any more detail about the crash, repeatedly saying that such questions should be referred to Ukrainian officials. An unnamed U.S. defense official told Reuters that the crash "did not appear to be the result of Russian fire" and said that other causes, including "pilot error" and "mechanical failure," were being investigated.

The preliminary report by Ukraine's General Staff said the F-16 had shot down four missiles during the attack, demonstrating "high efficiency." "During the approach to the next target, communication with one of the jets was lost. As it turned out later, the plane crashed, and the pilot was killed," the General Staff said.

Oleshchuk pledged to get to the bottom of the crash, and said that "no one has hidden anything (or is) hiding anything (now)." The details of the incident cannot be immediately made public for security reasons, Oleshchuk added. Ukraine received its first F-16s by the beginning of August, a year after its allies formed the fighter jet coalition at the NATO summit in Vilnius to support Kyiv with training and aircraft.

Just 10 jets have been delivered so far, according to media reports. Kyiv has been promised at least 79 F-16s from the Netherlands, Denmark, Norway, and Belgium.

BREAKING: F-16 fighter jet crashes in Ukraine, military confirms One of the F-16 fighter jets recently delivered to Ukraine crashed while repelling a Russian mass attack, Ukraine's General Staff said on Aug.

29.