At least two Russian troops were killed in a mortar attack on Moscow’s main airbase in Syria.

The Russian Defense Ministry confirmed the December 31 shelling of the Hmeimim airbase, from where Russia has launched thousands of sorties since its intervention to prop up the Assad regime in September 2015.

The Minsitry statement was vague on the attackers, saying they were “a mobile militant subversive group”. It denied reports, in the Kommersant newspaper, that seven warplanes were destroyed: “Russia‚Äôs air group in Syria is combat ready and continues to accomplish all its missions in full.”

Kommersant said four Sukhoi Su-24 bombers, two Sukhoi Su-35S fighter jets, one Antonov An-72 transport plane, and an ammunition depot were struck by “radical Islamists”. It also said up to 10 personnel might have been injured.

No rebel faction has claimed the attack. Rebels in Latakia Province have occasionally targeted the base with Grad rockets. Most are intercepted, meaning that a large volley would have to be launched for a chance of getting near targets.

An alternative theory from local sources is that a “corrupt” regime militia or a Syrian Army unit may have carried out the mortar assault.

On Wednesday, the Russian Defense Ministry said an Mi-24 helicopter crashed during a flight to an airfield near Hama. Both pilots were killed in the crash.

The Miinistry denied the Mi-24 was shot down.

Last month President Vladimir Putin made a sudden trip to the airbase to declare “victory” over “terrorists” in Syria’s 81-month conflict.