Western powers have condemned Syria for shooting down a Turkish military jet in a maritime border area, and have pledged to cooperate with Ankara in holding Damascus to account for Friday's incident.
In a statement issued Sunday, U.S. Secretary of State Hillary Clinton said the downing of the aircraft was a "brazen act" that reflects what she called Syria's "callous disregard for international norms, human life, peace and security."
British Foreign Secretary William Hague said London is ready to purse "robust" U.N. Security Council action against Syrian President Bashar al-Assad's government.
Italian Foreign Minister Giulio Terzi said Rome will play an "active role" in a NATO meeting scheduled for Tuesday to respond to Syria's attack on the jet. A NATO spokeswoman said Turkey requested the NATO meeting under the alliance's founding treaty, which commits all members to protect one another's security and borders.
Details of the aircraft incident are still not clear.
Turkish Foreign Minister Ahmet Davutoglu accused Syria on Sunday of shooting down the jet without warning in international airspace over the Mediterranean Sea. He said the jet's two pilots were on a training mission and inadvertently entered Syrian airspace for a brief period before leaving and being struck by Syrian fire several minutes later.
"The plane was not on a covert operation - it was on training mission to test radar, and the plane was unarmed," he said.
The Turkish pilots remain unaccounted for. Syria has said it fired on the jet because it was flying close to its coast, in violation of Syrian airspace.
Relations between Turkey and Syria have been tense since last year, when Ankara began criticizing Mr. Assad's deadly crackdown on an opposition uprising against his autocratic rule. Syria has criticized Turkey for hosting Syrian opposition forces. It accuses Turkish authorities of providing weapons and intelligence to the rebels.
Political observers in Turkey say the Turkish government's response to the attack on its aircraft has been restrained and measured so far. Ankara has promised to take strong, decisive and legitimate action once the facts of the incident are known.
In Syria Sunday, opposition activists said attacks by government and rebel forces killed at least 40 people across the country. They said fighting in the northeastern region of Deir el-Zour killed at least 13 people, while government shelling killed seven members of the same family in the northwestern province of Idlib.
Activists also said Syrian rebels captured a military post in the northern province of Aleppo, seizing a large amount of weapons. The activists' claims could not be independently confirmed.