Data on the flight recorder of the Russian war plane downed by Turkey last month has so far proved to be unreadable, Russia's military has said, according to BBC News.

It said the memory chips from the Su-24 bomber downed on the Syrian border were destroyed or badly damaged.

The Russians had hoped to prove the plane never entered Turkish airspace, as claimed by Ankara. Work will continue to try to retrieve the data, BBC says.

The incident sparked a major bilateral crisis, with Russia imposing sanctions.

The Russian military representatives presented their preliminary findings in Moscow on Monday, December 21, after opening the "black box" on Friday in the presence of international experts.

Some 13 of the 16 chips had been destroyed, while x-ray tests on the others showed extracting information would be impossible using normal methods.

However, the BBC's Sarah Rainsford in Moscow says further work will be done to see if other methods of retrieving the data are possible.

The Su-24 was shot down by F-16 fighters on 24 November.

Turkey insists that the jet, from the Russian air contingent deployed in Syria in support of President Bashar al-Assad, ignored warnings to leave its airspace.

Both crew members ejected but the pilot was killed, apparently by militants on the ground while the navigator was rescued.