MOSCOW - A Russian military transport plane crashed in Syria on Tuesday, killing all 39 people on board, Russian news agencies quoted the defense ministry as saying, sharply raising the death toll from the Kremlin's intervention in the Syrian war.President Vladimir Putin, who is running for re-election later this month, declared in December that the Russian mission in Syria was largely completed, but casualties continue to mount.The defense ministry was cited as saying that the plane, a Soviet-designed An-26, crashed at Russia's Hmeymim air base in Latakia Province and that initial information suggested the incident may have been caused by a technical fault."According to the latest updates, the An-26 transport aircraft, which crashed while landing at Hmeymim airbase, was carrying 33 passengers and six crew. All of them were members of the Russian Armed Forces," Interfax news agency quoted the defense ministry as saying."The crew and the passengers were killed."Originally, Russian news agencies cited the defense ministry as saying that 32 passengers and crew were killed in the crash, but this was later revised.Russian state TV cited military officials as saying the aircraft had not been brought down by enemy fire. It said the crash happened as the plane came into land at Hmeymim, and that it came down around 500 meters (550 yards) short of the runway. Putin, on an election campaign stop in the Ural mountains, was briefed on the crash via telephone by Defense Minister Sergei Shoigu and expressed his condolences to the relatives and colleagues of those killed, the Kremlin said.COSTLY CAMPAIGNRussia's armed intervention in Syria turned the tide of the conflict in favor of Putin's ally, Syrian President Bashar Assad. It has been portrayed inside Russia as a display of Russia's resurgent military might.It has, though, come at a cost.According to a Reuters tally based on official announcements before Tuesday's crash, 44 Russian service personnel have died in Syria since the operation started in September 2015.In one incident, in February this year, Syrian rebels shot down a Russian warplane. The pilot ejected and died on the ground in a gunfight with rebels.In addition to those deaths, in December 2016 a plane carrying a Russian military orchestra to Syria crashed in the Black Sea, killing all 92 people on board.Under a decree signed by Putin, the Russian authorities do not have to disclose all deaths of service personnel in Syria because they are classified as a state secret. The official toll does not include private military contractors who, according to people familiar with the deployment, are in Syria fighting in support of the Russian operation. Moscow denies they exist.About 300 men working for a Kremlin-linked Russian private military firm were either killed or wounded in Syria last month when their column was attacked by U.S.-led coalition forces, according to three sources familiar with the matter.Russian officials said a handful of Russian citizens may have been killed in that incident, but said they were not members of Russia's armed forces.Opinion polls indicate most Russians support the Syria operation. But Dmitry Gudkov, a former member of parliament and a member of the anti-Putin opposition, said the Kremlin had questions to answer about the campaign."How does the draw-down of forces look now? For the sake of what are these people dying? When will this senseless war end?" Gudkov said in a social media post.