Turkey threatened to take "decisive" action last night after Syria shot down one of its fighter jets, sparking fears that Nato could be drawn into a major confrontation with the Assad regime.
Recep Tayyip Erdogan, the Turkish prime minister, flew home from Brazil to hold an emergency briefing with his intelligence and military chiefs after radio and radar contact was lost with the aircraft as it conducted a mission close to the Syrian coast.
"Following the evaluation of data provided by our related institutions and the findings of the joint search and reduce efforts with Syria, it is understood that our plane was downed by Syria," his office said in a statement.
In an interview on Saturday morning, though, Turkey's president, Abdullah Gul, conceded that the jet fighter might have violated Syrian airspace.
"It is routine for jet fighters to sometimes fly in and out over (national) borders ... when you consider their speed over the sea," Mr Gul told the Anatolia news agency. "These are not ill-intentioned things but happen beyond control due to the jets' speed."
Mr Erdogan promised that Turkey's response would be both "decisive" and carried out with "determination". Although he did not divulge what steps he was contemplating, a senior member of his ruling party had earlier declared that if the aircraft was shown to have been shot down by Syria it would amount to a "declaration of war".
Syria confirmed that it had brought down the aircraft, saying in a statement: "Our air defences confronted a target that penetrated our air space over our territorial waters pre-afternoon on Friday and shot it down. It turned out to be a Turkish military plane."
In a sign that it was aware of the gravity of the situation, Syria seemed to be trying to repair the damage, deploying vessels to join a search and rescue operation to locate the aircraft's two pilots in the waters off its coast.
The incident represented the fulfilment of one of the international community's greatest fears after months of predictions that the Syrian conflict could easily burst its borders.
Western powers, and particularly the United States, are likely to come under pressure to support Turkey should it choose to retaliate with military force. Mr Erdogan's government has long warned that it would not tolerate any Syrian challenge to its security.
As a member of Nato, Turkey could potentially invoke Chapter V of the alliance's treaty which states that an attack on one state would be viewed as an attack on all signatories of the alliance.
But because the clause dictates that such an attack must be carried out on European or American soil, Mr Erdogan is unlikely to make such demands of his Western allies.
But he could well invoke Chapter IV of the treaty, which allows a member state to convene an emergency summit of the whole alliance if "the security of any of the parties is threatened".
Turkey came close to doing so in April after Syrian forces opened fire into its territory, wounding two Turkish nationals and two Syrians at a refugee camp close to the borders.
It was persuaded not to do so by the United States, but is likely to be less malleable now. In return for agreeing to allowing Saudi and Qatari funnel weapons to the rebels through its territory, Mr Erdogan sought and received assurances that America would protect Turkey from any Syrian backlash, according to Western officials.
Turkey, which has been at the forefront of regional efforts to oust Mr Assad and has given sanctuary to rebels seeking his overthrow, could also try to revive previous efforts to win international support for a buffer zone in Syria's border regions.The jet incident came as the Syrian government accused its rebel foes of carrying out a "massacre" of Mr Assad's supporters after the emergence of grisly video footage showing more than a dozen bloodied and mutilated corpses.