Leading article: An omen of how Syria's conflict could spread

When the Syrians shot down a Turkish military jet at the weekend, Turkey's government responded with commendable moderation. But the temperature rose yesterday when the country's Deputy Prime Minister, Bulent Arinc, revealed that Syria had gone on to fire on a plane sent to look for the missing pilots and described this as "a hostile act of the highest order". Turkey is now threatening a military response against any Syrian forces approaching the long border between the two countries and has revised its military rules of engagement.

This is an alarming development. Since the start of the uprising against Bashar al-Assad, there has been concern that the violence could spark a wider regional conflict. Relations between Turkey and Syria have long been strained, with recurrent disputes over the border, water, Damascus's support for Kurdish rebels and, most recently, the influx of Syrian refugees.

But just as significantly Turkey is a member of Nato. Ankara has approached the alliance now under Article 4 of the Nato Treaty which allows any member state to demand a Nato meeting if it believes its "territorial integrity, political independence or security" is threatened. And it is only the second time in Nato's history that a member state has invoked Article 4. In response Nato has condemned Syria's attack "in the strongest terms" and said the alliance's 28 members will "stand together with Turkey in the spirit of strong solidarity". The incident, it said, was another example of the Syrian authorities' disregard for international norms.

Referring the issue to Nato is a double-edged sword for Turkey. It summons the power of the alliance, but it also binds Ankara to take the advice of Nato members among whom there is still no appetite for military intervention in Syria. Nato works by consensus and all members must approve any action. Yet Turkey has pointedly refrained from invoking Article 5 of the Treaty by which an attack on one member is considered an attack on all.

What this means is that, for all the tough talk of Turkish politicians, its government is limiting its response as yet to a diplomatic rather than a military one. Such caution is prudent. For the downing of the military jet has shown that the international community is here faced with a problem far tougher than that posed by Libya or even Iraq. Syria's air defences are far more extensive than those of Libya, and even these required US air power to destroy.

But if the prospect of Western military intervention in Syria remains remote, what this episode highlights is the way the Syrian crisis is heightening existing volatility. Yesterday there were reports of fighting in the suburbs of Damascus where part of Syria's formidable chemical weapons arsenal is believed to be stored. Both Israel and the US have said privately they would be forced to act if these facilities became insecure. As the bloodshed grows, and the Assad regime becomes more desperate, so the risk mounts that some unforeseen event might ignite a broader conflagration.

Official Nato military intervention is unthinkable without the support of either the Arab League or the UN Security Council, where Russia has a veto. But incidents like the shooting down of the Turkish plane illustrate the knife-edge on which regional security is poised. The nightmare prospect of a conflict involving Syria, Turkey, Israel and Iran becomes more credible with each passing day.

Russia is the key to unblocking the impasse. The West should concentrate now on providing Moscow with assurances that its geopolitical interests will be respected in Syria so long as it assists in a regime change which sees the back of the blood-stained President Assad.