SU-24 AND SU-25

What they are: Making up the core of the force deployed in Syria are SU-24 fighters (known to Nato as Fencers) and SU-25 ground-attack jets (Frogfoots), two workhorse warplanes. The battle-tested models, which date back to Soviet times, were familiar sights over battlefields such as Chechnya and Ukraine.

Why they may matter: The Syrian airforce also has SU-25s, but Russia's planes have been upgraded so they are able to fight at night, a capability the Syrian planes lack.

SU-30M FLANKER

What they are: Highly manoeuvrable jets designed mainly for dogfights.

Why they may matter: They may be intended to warn the West away from trying to create a no-fly zone without Russian acquiescence.

SU-34 FULLBACK

What they are: A newer and more sophisticated ground-attack aircraft.

Why they may matter: This is their first known combat deployment, and they may have been sent, specifically, to test them in combat conditions.

They have longer range and better air-to-air combat capabilities than the Frogfoots. The Russians reportedly used them in a strike in eastern Syria last Friday, flying in a zone that is also traversed by US jets bombing ISIS targets.

PANTSIR S1 GREYHOUND SYSTEM

What they are: Radar-guided anti-aircraft artillery and missile systems mounted on tractor-trailer truck beds.

Why they may matter: They are capable of defending only a small zone around the airfield of a Russian airbase, so they have little strategic impact. But they serve as a warning to other airforces to stay well clear of the Russian base.

NAVAL FORCES

What they are: Russia has positioned a naval battle group in the eastern Mediterranean Sea, including ship-based long-range surface-to-air missiles, according to a defence ministry spokesman.

Why they may matter: Russia's air base is in the region, near Latakia, and Russia has long had a naval supply and maintenance base at Tartus. Among other capabilities, the naval forces can control large stretches of airspace. The Russian authorities have cautioned civilian aircraft to avoid the area, even at high altitudes. It is a warning, needless to say, to the US Air Force as well.

NEW YORK TIMES