* Effective air defences supplied by Russia
* Relations between Ankara and Damascus already tense
* Reminder of difficulties of any foreign intervention
By Peter Apps, Political Risk Correspondent
LONDON, June 24 (Reuters) - While the exact circumstances remain far from clear, the shooting down of a Turkish warplane shows Syria's military to be capable, extremely jumpy and increasingly drawn into confrontation with its most powerful neighbour.
That could prove a major deterrent for Western powers in particular, who want President Bashar al-Assad gone but are unwilling to risk troops or aircraft in a military intervention. Equally, they are wary of triggering a wider regional war.
Turkey says its F4 Phantom reconnaissance jet was engaged in testing the domestic Turkish radar system when it entered Syrian airspace by mistake. But Ankara is adamant it was firmly back over international waters when it was attacked without warning. Syria says the aircraft was firmly within its airspace and approaching its coast low and fast.
Exactly what the jet's true mission was remains far from clear. While Turkey says it is not unusual for planes to drift across national boundaries when on missions or exercises, the dangers of straying into Syrian airspace at such a time of tension would have been very apparent.
Having complained repeatedly that Syria's escalating conflict was crossing its borders, with thousands of refugees fleeing and occasional cross-border Syrian artillery and small arms fire, Turkey is widely believed to be increasing its support for Free Syrian Army (FSA) rebels on its soil.
The downed jet, some believe, may have been on a reconnaissance mission for the rebels or possibly trying to probe Syria's Russian-made radar and air defences. With Western and Arab powers increasingly actively trying to bring down Assad, some analysts say almost anything is possible.
"What all this tells us is that there are a lot of "fishy" tactics and strategies going on in the region, with numerous players behind many curtains," said Hayat Alvi, lecturer in Middle Eastern studies at the U.S. Naval War College.
"The Syrian military has reason to be jumpy, given these circumstances. Nonetheless, the idea that it would be in Turkey's and Syria's respective national interests to engage in military conflict with each other is not plausible. Both sides would have too much to lose, and very little to gain."
Friday's incident underlined the fact that should foreign powers hope to repeat the kind of military intervention that toppled Muammar Gaddafi in Libya, it would first require an overwhelming assault on Syria's air defences.
Foreign militaries may well now feel they have little choice but to keep a more respectful distance from Syria's borders, aware that even sophisticated high flying U.S. spy planes or pilotless drones could prove vulnerable.
"It's clearer that this was... a reconnaissance jet - reinforcing my view that this was a surveillance flight, and therefore suggestive of a bolder Turkish effort to step up the pressure and assist rebels," said Shashank Joshi, senior fellow and Middle East specialist at the Royal United Services Institute.
Turkey denies the jet mission had anything to do with the situation in Syria. The two countries initially cooperated in the search for the aircrew and wreckage in what appeared to be a deliberate strategy to avoid further escalation.
Ankara says the aircraft was clearly marked as Turkish and Syrian claims that they did not know its nationality when they fired were not convincing.
"Understanding the circumstances of the incident is crucial in informing Turkey's response especially as the region may ignite with hasty knee-jerk reactions," said Anthony Skinner, regional analyst for political risk consultancy Maplecroft. "This is something which (Turkish Prime Minister Tayyip) Erdogan, who is not one to pull his 'rhetorical punches', clearly understands."
So far, Turkey has stopped short of explicitly threatening a military response. It has said it will formally consult its NATO allies under the alliance's Article Four, avoiding invoking the common defence clause under Article Five that could request other members support in retaliation.
Some believe the entire incident may simply be the result of mistakes on both sides. According to the Syrian military, the jet was shot down by direct anti-aircraft gunfire rather than by anti-aircraft missiles, which have a much longer range.
If the jet were only in range four minutes or less, the gunners might have fired without any time to consult senior commanders or Damascus itself.
"The assumption here is that the F4 strayed into Syrian territory," said Henri Barkey, international relations professor and Middle East specialist at Lehigh University in Pennsylvania.
"The Syrians are clearly quite nervous and are likely to interpret any action, however innocent, as hostile. My guess is that anti-aircraft units have been given orders to shoot at anything that crosses into Syrian airspace. Reports of Turkish arms support for the insurgents also feeds the paranoia of the regime."
With Assad under mounting strain, it is even possible the Syrian gunners thought they were firing on one of their own aircraft to stop it fleeing the country. Earlier this week, a Syrian air force colonel flew his aircraft to Jordan to defect.
Syria bought new air defence systems from Russia, along with the training that goes with it, after Israeli jets penetrated its airspace in 2006 to "buzz" Assad's summer palace in 2006 in the apparent hope Damascus would pressure militant group Hamas to free kidnapped soldier Gilat Shalit.
Defence experts say the systems purchased were far from the best available, and were unable to prevent another Israeli raid the following year that destroyed a suspected Syrian nuclear weapons site; although there were some reports that raid was only enabled by a cyber attack that blinded Syrian radar.
As Gulf states in particular ramp up their support for opposition fighters, Syria's air defence systems also make the kind of parachute weapons and supply drops that were vital to rebel states in Libya largely impossible.
In principle, if Turkey decided it appropriate, it could demand military support from the rest of NATO or simply take its own retaliatory action against Syrian military targets.
But few believe that is likely, at least for now.
"NATO as a body is likely to be cagey in offering its support, and NATO's collective-defense clauses do not necessarily cover all contingencies, particularly if you fly into someone else's airspace," said Joshi at the Royal United Services Institute.
Ultimately, whether by accident or design, some argue the entire incident has given Damascus the chance to send an explicit warning to the rest of the world to back off.
"Turkey is unlikely to undertake a direct military response against Syria without support from Washington," said Fadi Hakura of Britain's Chatham House think tank.
"Washington does not seem keen for a military escalation prior to the presidential elections.
"Syria has refused to apologise... sending a clear message... that it has the wherewithal and inclination to defend its airspace and territory against anti-Assad opponents." (Additional reporting by William Maclean; editing by Ralph Boulton) (Reporting By Peter Apps)