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.