A Russian military journalist has published photos of Russian warplanes which are believed to have been damaged by rebel shelling in Syria.
Roman Saponkov posted them on social media after reports of the attack.
Russia's defence ministry acknowledged a 31 December rebel mortar attack on Hmeimim airbase but denied any jets had been disabled.
It also confirmed two servicemen's deaths. Russia's Kommersant newspaper said seven jets had been destroyed.
Kommersant quoted two unnamed "military-diplomatic sources" on the previously unreported incident - apparently the heaviest attack yet on the airbase near Latakia, in north-western Syria.
The attackers were described as "Islamist extremists".
According to Kommersant, in the attack "at least seven planes were destroyed: four frontline Su-24 bombers, two Su-35S multi-role fighters and an An-72 transport plane".
Posting on the social media site vKontakte, Saponkov listed the losses as: six Su-24s; one Su-35S; one An-72; one An-30 spy plane; and one Mi-8 helicopter.
Only one Su-24 and the Su-35S had later been put back into service, he added.
Speaking to the BBC, he said he had got the photos from a reliable anonymous source.
Saponkov pointed out that the number 29 on a jet's damaged tail fin also appeared on Russian state TV video of jets at Hmeimim in December.
Read more on the Syria conflict:
Hmeimim is Russia's main base for air strikes on rebel groups in Syria - strikes that have enabled President Bashar al-Assad's forces to recover much lost ground.
Russian air strikes are said to have killed many civilians though Moscow insists that it only targets rebel "terrorist" fighters.
The Russian military says it has now beefed up security at Hmeimim. The base already has long-range anti-aircraft missiles but rebels must have got close to the perimeter to target planes with mortars.