NATO said it would meet to discuss member state Turkey's accusation that Syria shot down one of its warplanes in international airspace and not inside its own territory, as Damascus claims.
NATO said it would hold an emergency meeting on Tuesday following a request from Turkey that invoked Article Four of the alliance's founding treaty, which covers threats to member states' security.
Turkey has already acknowledged that its fighter jet might at some point have entered Syrian airspace. But after an initially cautious response, Ankara toughened its rhetoric on Sunday.
Turkish Foreign Minister Ahmet Davutoglu told Turkey's TRT television that at the time it was shot down, their plane was in international airspace, 13 nautical miles from Syria.
Syria had given no warning before opening fire, he added.
The fighter had been on an unarmed training mission to carry out a radar system test, and both pilots are still missing.
"The Syrians knew full well that it was a Turkish military plane and the nature of its mission," said Davutoglu.
"Nobody should dare put Turkey's (military) capabilities to the test," he warned.
"We will bring this affair before public opinion and international law in the name of Turkey's honour."
Syria has acknowledged shooting down the F-4 phantom jet after it violated its airspace, but insists it only identified it as a Turkish fighter after the fact.
"What happened was an accident and not an assault as some like to say," Syrian foreign ministry spokesman Jihad Makdissi told the Al-Watan pro-government daily on Sunday.
But UN Security Council member Britain warned that Syrian President Bashar al-Assad's regime "should not make the mistake of believing that it can act with impunity."
"It will be held to account for its behaviour," Foreign Secretary William Hague said.
US Secretary of State Hillary Clinton condemned a "brazen and unacceptable act" and promised US assistance in investigating the incident.
"It is yet another reflection of the Syrian authorities' callous disregard for international norms, human life, and peace and security," Clinton said in a statement.
Italy, another NATO member, also condemned Syria over the incident and Foreign Minister Giulio Terzi said they would "actively participate" in the NATO meeting.
In his comments to Al-Watan Sunday, foreign ministry spokesman Makdissi tried to pour oil on the troubled waters.
"Syria was merely exercising its right and sovereign duty and defence," he said.
"There is no enmity between Syria and Turkey, but political tension (exists) between the two countries."
Turkish-Syrian relations have been strained by Prime Minister Recep Tayyip Erdogan's outspoken condemnation of the Assad's regime's bloody crackdown, which activists say has killed more than 15,000 people since March 2011.
Already Saturday, UN chief Ban Ki-moon had expressed his "deep concern" about the incident, particularly about the "potential serious implications" for the region, spokesman Martin Nesirky said.
The fighting inside Syria claimed scores more lives on Sunday.
The official SANA news agency reported that regime troops had killed "dozens of terrorists" after engaging them as they attacked people in the Jabaliye neighbourhood of Deir Ezzor.
The Syrian government uses the term "terrorists" to describe the rebels.
The Syrian Observatory for Human Rights said at least 91 people had been killed on Sunday in Syria, including 59 civilians.
The Observatory also reported that following an attack on an artillery battalion in Aleppo, a number of soldiers had defected, taking with them a large quantity of weapons.
And it said that rebels had shot down a Syrian regime helicopter near the Jordanian border.
"This is one of the bloodiest weeks in the conflict," Rami Abdel Rahman, head of the Observatory, said.
"It's like we are in a war. Sometimes when two countries are at war, not even 20 people are killed a day. But now in Syria it has become normal to have 100 killed each day."
Australia on Monday announced fresh sanctions against Syria, restricting or prohibiting trade across entire sectors.
"The Assad regime continues to show its unwillingness to negotiate a ceasefire and bring an end to Syria's bloodshed," Foreign Minister Bob Carr said in a statement.
In an opinion piece for The Australian newspaper, Carr called for Moscow to take a lead role in forcing Assad out.
But there were reports that Russia was to try again to send a Russian ship to make a controversial delivery of attack helicopters to Syria.
A first journey ended after exposure by the US authorities led the ship's British insurer to withdraw cover.
But an unnamed Russian diplomat said the ship, the Alaed, would soon try again to deliver the aircraft.
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