The Russian military says the "black box" from a warplane shot down by Turkey last month is too badly damaged to be deciphered, at least without additional work.
Russian authorities have expressed hope that the recorder would yield information supporting Moscow's claim that the Su-24 bomber did not enter Turkish airspace before it was shot down near the Turkey-Syria border on November 24.
Turkey says the Russian crew ignored multiple warnings and violated the country's airspace.
"Retrieving the information and a read-out of flight data...has proven to be impossible because of internal damage," Sergei Baimetov, the Russian Air Force's deputy head of flight safety, said on December 21.
He said Russia would seek help from specialists and that "a lot of time" would be needed to try to get information from the black box.
One of the Russian plane's crewmen was killed after parachuting from the plane over northern Syria, and a rescuer was killed during the search.
The downing has severely strained relations between Russia and Turkey, and complicated diplomatic efforts to end the nearly five-year civil war in Syria.
With reporting by Reuters and TASS