Turkey is to demand backing from its Nato allies on Tuesday at a special meeting called after Syrian troops shot down a Turkish warplane last week in an incident Damascus said was self-defence but which Ankara branded an "act of aggression".
It is only the second time in Nato's 63-year history that it has convened under Article 4 of its charter which provides for consultations when a member state feels its territorial integrity, political independence or security is under threat.
Turkey rejected assertions from Damascus that its forces had no option but to fire on the F-4 jet as it flew over Syrian waters close to the coast on Friday.
In a letter to the UN Security Council, Turkey condemned the "hostile act by the Syrian authorities against Turkey's national security", saying it posed "a serious threat to peace and security in the region".
Deputy Prime Minister Bulent Arinc told a news conference Syria's actions "would not go unpunished".
The incident has further heightened tensions between Turkey and Syria already strained to near breaking point over the 16-month uprising against Syrian President Bashar al-Assad's rule.
Though not known for his restraint, Turkish Prime Minister Tayyip Erdogan has so far been measured in his response, perhaps due to Western reluctance to commit to any military action and wary himself of anything that could trigger a regional sectarian war.
European Union foreign ministers called on Monday for Turkey to show restraint, saying they would increase pressure on Assad.
"Military intervention in Syria is out of the question," said Dutch Foreign Minister Uri Rosenthal.
After a seven-hour Turkish cabinet meeting in which an air force chief gave a detailed briefing on Friday's incident, Arinc said Erdogan would make an announcement on Syria on Tuesday.
Syria's description of the event as an act of self defence, though tempered with commitment to a "neighbourly relationship", seemed likely to further anger Ankara.
Act of Defence
"The plane disappeared and then reappeared in Syrian airspace, flying at 100 metres altitude and about 1-2kms (0.6-1.2 miles) from the Syrian coast," Syrian Foreign Ministry spokesman Jihad Makdissi told a Damascus news conference.
"We had to react immediately, even if the plane was Syrian we would have shot it down," he said. "The Syrian response was an act of defence of our sovereignty carried out by anti-aircraft machinegun which has a maximum range of 2.5 km."
Syria warned Turkey and Nato against retaliating.
"Nato is supposed to be there to strengthen countries," said Makdissi. "If their meeting is for hostile reasons (they should know that) Syrian land and waters are sacred."
The only other time Nato has convened under Article 4 was in 2003 to discuss the Iraq war, again at the request of Turkey.
The unarmed reconnaissance jet had briefly entered Syrian airspace as it approached land after patrolling the eastern Mediterranean, Arinc said, but was warned by Turkish radar controllers and immediately left and turned again out to sea. It then made another approach to land when it was shot down 13 miles off the coast in international airspace, he said, out of the reach of Syria's anti-aircraft guns
"According to the data in our hands, it points to our plane being shot by a laser or heat-guided surface-to-air missile. The fact our plane was not given an early radar warning, suggests it was not a radar-guided missile," said Arinc.
By invoking Article 4 of the Nato charter, rather than Article 5 which calls for military action, Turkey has signalled it wants action against Syria short of armed intervention.
"Ankara itself has been averse to consider military action against Syria so far. So it is likely that the invocation of Article 4 is designed to put more diplomatic pressure on Assad," said Clara Marina O'Donnell, a fellow at the Brookings Institution in Washington.
In shell-shattered districts of Homs, the heart of the uprising against Assad, rebels battled troops as aid workers tried to evacuate civilians. Turkish television reported the desertion of a Syrian general and other officers across the border.
The International Committee of the Red Cross (ICRC) said it was again trying to arrange a safe evacuation of trapped civilians from Homs. But anti-government activists reported heavy shelling on central districts, including Jouret al-Shiyah and Al Qarabis. Video showed detonations and machinegun bursts from the skeletal remains of abandoned apartment blocks.
The activist Syrian Observatory for Human Rights said Assad's troops carried out raids and arrests in areas still under army control, and heavy fighting between government forces and rebel fighters was reported in the opposition centres of Idlib, Deir al-Zor and Deraa, the birthplace of the uprising.
The United Nations has said more than 10,000 people have been killed by government forces, while Syria has said at least 2,600 members of the military and security forces have been killed by what it calls foreign-backed "Islamist terrorists."
The intensification of the fighting has raised fears in Turkey of a flood of refugees and a slide into ethnic and religious warfare that could envelop the region. Ankara, like the West, is torn between a wish to remove Assad and the fear that any armed intervention could unleash uncontrollable forces.