President Vladimir Putin first launched a military intervention in Syria in 2015, turning the tables in the multi-front civil war in favour of ally Assad.

The regime and its Iranian and Russian allies increasingly appear to be on the path to victory in Syria's brutal seven-year civil war, while the West has been reluctant to engage militarily.

Here is what you need to know about the Russian army's presence in Syria.

- How many troops? -

The precise number of Russian troops fighting in Syria is not known.

Putin said in December that some 48,000 Russian servicemen have taken part in the campaign.

He has ruled out dispatching ground troops in Syria, making the air force the mainstay of Moscow's Syria campaign.

Officials have acknowledged the presence of Russian advisors and military police in Syria, however.

Independent Russian military expert Pavel Felgenhauer estimated late last year that "up to a thousand" military police and special forces are fighting alongside regime troops.

In March, the defence ministry said a total of 2,954 Russian troops in Syria had voted in presidential polls, giving an idea of current numbers.

In December Putin made a surprise visit to Russia's Hmeimim airbase in Latakia, Syria where he ordered a partial pullout of the country's troops, saying their task had been largely completed.

Russia said the airbase, set up in 2015, would remain in operation permanently, while its naval facility in the Syrian port of Tartus would be expanded.

Three battalions of military police and officers of the Russian Centre for Reconciliation would remain in Syria, Defence Minister Sergei Shoigu said.

Putin first said in March 2016 that the country's military campaign in Syria was coming to an end.

Today few believe Russia is leaving the war-torn country any time soon.

- Mercenaries -

Officials and observers also point out the presence of a "shadow army" of Russian mercenaries in Syria, including those working for a private military company called Wagner.

Military expert Felgenhauer last year estimated the number of mercenaries at 2,000 to 3,000.

Moscow in March acknowledged five Russian nationals were killed and "dozens" wounded in US strikes on an area where no official troops were present, saying they all were in Syria "on their own initiative."

Various media reports in Russia and the United States said Wagner was being funded by one of Putin's allies Yevgeny Prigozhin, a Saint Petersburg businessman who previously carried out catering contracts for Russia's military and the Kremlin.

He has been charged by a US court and hit by Washington sanctions over setting up an internet "troll factory" that is accused of attempting to influence the 2016 presidential election in favour of Donald Trump. He has denied involvement.

- How many casualties? -

Russia has officially acknowledged 84 military losses in the Syrian war, including 39 killed in a plane crash at Hmeimim in March that was not caused by military action.

- Fighter planes, bombers -

Russia rules the skies in Syria and the role of its air force there has been celebrated at home.

Bomber planes such as the Tu-22 and Tu-160 have also flown from Russia to hit targets in Syria.

- Navy, aircraft carriers -

Russian warships and submarines have also played a prominent role backing up the bombing campaign in Syria, firing missiles at Islamic State group targets from the Mediterranean.

The Hmeimim airbase on the western coast and the naval facility in Tartus have been protected by S-300 and S-400 air missile defence systems.

Russian ships, such as Moscow's only aircraft carrier, the Admiral Kuznetsov, have also completed missions in Syria. The Kuznetsov is now out of action for repairs set to last 2 to 3 years.

Russian ships are passing back and forth through the Bosphorus every couple of days.

The Russian defence ministry has previously said it will fire at any US craft that strikes government areas of Damascus, where Russian officials are also based.