Russia has repeatedly used its UN Security Council veto to support Bashar al-Assad throughout the four and a half year-long war, which is believed to have claimed some 250,000 lives. Russia has also been a long-term supplier of arms to the Syrian government, something it now justifies by the need to fight Islamic State of Iraq and the Levant (Isil).
Speculation is growing that Russia has significantly expanded its involvement in recent months, including with deliveries of advanced weaponry, a raft of spare parts for existing machines, and the deployment of increasing numbers of military advisers and instructors.
Last week Syrian state television released images showing an advanced Russian-built armoured personnel carrier, the BTR-82a, in combat. Videos have also appeared in which troops engaged in combat appear to shout instructions to one another in Russian.
Last week the Israeli daily Yedioth Ahronoth cited western diplomatic sources saying that Russia was on the verge of deploying "thousands" of troops to Syria to establish an airbase from which the Russian air force would fly combat sorties against Isis.
Russian analysts called the Yedioth report far-fetched, pointing to Russian wariness of repeating the American experience in Iraq and the current strain on the Russian military from a covert war in Ukraine.
Most government-connected analysts have previously insisted that Russia's support for Mr Assad is "strictly political", and have dismissed reports of military involvement as "madness".
"It is a canard. A deployment of that size would require approval from the Federation Council [Russia's upper house of parliament]," said Yevgenny Buzhinsky, a retired Russian general who now heads the PIR analytical centre in Moscow. "As far as I am aware any advisers there do not engage in combat."
But Mr Putin's comments chime with experts who say the Russian government would be willing to supply substantial logistical support and advice even if it shies away from large-scale intervention.
"Such things are kept very secret, but there is definitely an adviser and instructor mission there, possibly numbering in the hundreds," said Pavel Felgenhaeur, an independent commentator on Russian military affairs.
"It definitely includes technical advisers and engineers to maintain sophisticated military equipment, and marines to protect them. There is no way Assad's jets could still be flying after four years of war without Russian technical assistance," he said.
Mr Felgenhauer said it was "quite conceivable" that members of the advisory mission occasionally found themselves in combat or had even suffered casualties.
A senior Syrian regime military official who defected in 2012 told the Telegraph that he had personally worked alongside Russian officers, but that in his experience they were there "as experts, not fighters".
"Most of the operations room and many of the defence lines are planned by Russian experts, so there are extra technical personnel now. They are mainly in Damascus," the defector said, citing former colleagues who are still serving with the Assad government.
The reports of increased support follow a recent diplomatic offensive in which Russia has attempted to persuade western and Arab governments, as well as members of the Syrian opposition, that Mr Assad should be part of a national unity government and an international alliance to fight Isil.
Mr Putin said on Friday that Mr Assad had agreed to such a deal, "right up to the point of holding early parliament elections and establishing contacts with the so-called healthy opposition and engaging them in governing".
However, Western governments and Syrian rebel leaders have so far insisted that there is no place for Mr Assad in post-war Syria.
In a war riven with barbarity, much of it led by fighters from Isil, Mr Assad's regime still remains the biggest killer of civilians.
Khaled Khoja, the chairman of Syria's opposition National Coalition, said after a recent meeting with Russian officials in Moscow that there was no question of sharing power with Mr Assad.
Another option debated in foreign policy circles would involve Mr Assad stepping down to be replaced by a mutually acceptable successor.
Russia is said to oppose this view, believing that removal of Assad would lead to the complete collapse of Syria as a state.
The Pentagon said on Friday that it had seen reports of Russia deploying troops and aircraft in Syria, and was "monitoring the situation closely".
As for the possibility of Russia joining the coalition against Isil, Pentagon spokesman Peter Cook said the US would "welcome the opportunities for others to join the fight".
However, Mr Cook said "the Assad regime cannot be a partner against the terrorism that it has both curated and then failed to confront effectively".
