Investigators in Moscow say they are unable to retrieve information from the damaged black box of a Russian warplane shot down by Turkey last month. This may make it difficult for Russia to determine whether its jet was flying over Turkish territory.
The Russian Air Force's deputy head of flight safety, Sergei Bainetov, says 13 of the flight recorder's 16 microchips had been destroyed and that those remaining were damaged.
Russia will now seek help from specialists. Bainetov says "a lot of time" will be needed to try to achieve a breakthrough.
Turkey shot down the Su-24 bomber at the border with Syria on November 24th. It says the plane violated its airspace for 17 seconds despite repeated warnings. Russia insists the plane never entered Turkish airspace.
Relations between the two countries have been badly strained ever since.