New satellite imagery appears to reveal extensive damage to a strategically significant airbase in central Syria used by Russian forces after an attack by so-called Islamic State (IS).
Four helicopters and 20 lorries were destroyed in a series of fires inside the T4 base last week, the images from intelligence company Stratfor suggest.
The cause of the fires is unconfirmed.
A pro-Kremlin website said the helicopters had been used by “used by both Russian and Syrian air forces”.
Russia has not officially commented on the incident.
A Russian opposition website quoted Syrian sources as saying “a large fire in the Syrian part of the T4 airbase spread to the fleet of vehicles, and after a fuel tank exploded four Russian helicopters nearby went up in flames”.
“The cause of the fire is being established,” it added.
A serious loss, by Frank Gardner, BBC security correspondent
Regardless of what triggered the fire that destroyed four Russian attack helicopters at their central Syrian base, this is the most serious loss for the Russians so far in their engagement against IS.
When the Kremlin deployed forces to Syria last September it was initially cautious about getting drawn into a ground war, wary of taking the mounting casualties that became a feature of its exhausting 10-year campaign in Afghanistan.
But after announcing a strategic withdrawal earlier this year Russia now seems to be getting ever more involved in supporting the forces of Syria’s President Bashar al-Assad.
The destruction of so much Russian hardware at the T4 airbase, 40 miles (60km) from Palmyra, has revealed some of the extent of that involvement.
Palmyra is deep inside the Syrian desert, far from the coastal and border battlefields of north-west Syria, where the Russian air force has been most active.
The question now is whether Russia will react to this loss by replacing, increasing or scaling back its presence in Palmyra.
Speculation that the fire was accidental was fuelled by the first report of the incident, which came from Amaq, a news agency linked to IS, says BBC Arabic’s Syria correspondent Rami Ruhayem.
“Burning of four Russian attack helicopters and 20 trucks loaded with missiles inside T4 airport in eastern Homs [province] as a result of a nearby fire,” the report said, without identifying the cause.
On the same day, IS released an image it said showed one of its fighters firing Grad rockets at T4, also known as Tiyas.
“What the imagery tells us is that first of all this was not an accidental explosion, as some of the rumours kept saying,” Stratfor military analyst Sim Tak said.
“It shows very clearly that there are several different sources of explosions across the airport, and it shows that the Russians took a quite a bad hit.
“An entire combat helicopter unit was wiped out – four helicopters in total – as well as some damage to some of the Syrian planes on the airport, and also very notably a logistic depot, likely one that was being used to supply those specific combat helicopters.”
Mr Tak described Amaq’s account as “very accurate”, and suggested the helicopters and depot were destroyed by IS attacks.
He said it was unclear why IS had not officially said it had caused the destruction.
“In the past IS has claimed similar attacks, they have even videotaped the attacks themselves.
“In this case, we haven’t seen any of those materials come out yet. One possibility is that by making the statement they were intending to claim it while not necessarily phrasing it that way.”
Mr Tak said it “would really be a marginal, almost non-existent chance for this to be accidental”.
In addition to the Russian losses, the area where it happened is strategically significant, Rami Ruhayem says.
The province of Homs stretches out into the Syrian desert towards Raqqa, the capital of the “caliphate” proclaimed by IS in 2014, and Deir al-Zour, on the way to the border with Iraq, he notes.
As it tries to shake the government’s grip on Homs, IS is facing a diverse range of factions on the ground.
The Iranian Al-Alam news website carried reports on 16 May that “Syrian allied forces” had cut off IS supply lines between Raqqa and Homs by taking over four hills overlooking the Shaer gas fields held by IS.
But Mr Tak counters that IS supply lines to Raqqa are holding.
“The regime and the Russians are facing notable challenges in trying to disconnect those two.
“Right now though, the emphasis seems to be more on moving towards Deir al-Zour rather than messing with Raqqa.”