Dozens of Russian military vehicles and attack helicopters deployed to Syria may have have been destroyed in an apparent attack by Isis.
Images released today by Stratfor, the American intelligence think-tank, show what appear to be the blackened remains of four attack helicopters and dozens of lorries at an airbase used by Russia near the town of Tiyas.
The images appear to confirm an earlier claim by Isis (Islamic State) that four helicopters and 20 trucks had been destroyed in a fire at the base.
That would represent Russia's worst losses in terms of materiel since President Vladimir Putin ordered intervention in Syria in September.
Russia dismissed the Isis claim as "propaganda", said said the images showed damage inflicted during months of heavy fighting for the base before Russian forces occupied it earlier this year.
All Russia helicopters deployed to Syria "are performing their planned missions to destroy terrorists", said Major General Igor Konashenkov, a Russian military spokesman in Moscow.
"The rumours about the destruction of Russian military helicopters and two dozen lorries were authored by Isis propagandists, who unsuccessfully tried to 'sell' this alleged 'news' 10 days ago," he said.
However, analysts at Stratfor said the new images show the Isis claims were actually "very accurate" and suggest the fire was caused by a terror attack rather than an accident.
"The imagery tells us this was not an accidental explosion. It show that the Russians took quite a bad hit. An entire helicopter unit was wiped out," Sim Tack, an analyst at Stratfor, told the BBC.
"That first statement shows that Islamic State had the knowledge about what exactly was destroyed in the attack," he added.
Syrian sources told Russian media that there had indeed been a fire at the base, but it was unknown what caused it. The sources said no one was hurt in the blaze.
Isis put out a statement though its affiliated al-Amaq news channel on the day of the attack on May 15, claiming a fire at the base destroyed four Russian helicopters and 20 trucks carrying rockets.
The Russians built up their presence at Tiyas, a former Syrian military air base, after the liberation of Palmyra in March. Just weeks earlier, Putin had announced Moscow was winding down operations in Syria and recalled many of its aircraft.
Isis has mounted a series of high profile attacks in recent weeks in an apparent bid to demonstrate it can still take the offensive despite the loss of Palmyra.
At least 148 people were killed in coordinated suicide bombings claimed by the group in government-held territory on Tuesday. The attacks included blasts in the towns of Tartus, home to a Russian naval facility, and Jableh, near a Russian-operated air base.