Syria confirmed it had shot down a Turkish reconnaissance plane because the plane entered its airspace, insisting it was "not an attack" as both sides desperately tried to de-escalate the episode before it exploded into a regional conflagration.
Turkey threatened to retaliate but did not say what action it would take as it searched for the aircraft's two missing pilots.
The downed plane heightened tensions between two countries that had been allies before Syria's 15-month violent uprising, and signalled that the violence gripping Syria is increasingly bleeding outside its borders.
Germany and Iraq were among the countries urging restraint in the region.
Syria and neighbouring Turkey had cultivated close ties before the Syrian revolt began in March 2011, but since then Turkey has become one of the strongest critics of Syria's regime.
Turkey hosts civilian and military Syrian opposition groups, including hundreds of army defectors who are affiliated with the Free Syrian Army and collect food and other supplies to deliver to comrades on smuggling routes.
Turkish authorities also suspect Damascus, which was collaborating with Turkey in its fight against autonomy-seeking Kurdish rebels, is now turning a blind eye to Syria-based Kurdish fighters who belong to the Kurdistan Workers' Party, or PKK, considered a terrorist organisation in the US and Europe.
The plane, an unarmed F4, went down in the Mediterranean Sea about eight miles from the Syrian town of Latakia, Turkey said. Syria claimed the jet violated its air space over territorial waters, penetrating by about a kilometre. It said Syrian forces only realised it was a Turkish jet after firing at it.
In a telephone interview with Turkish TV news channel A Haber yesterday, Syrian Foreign Ministry spokesman Jihad Makdissi said the downing was "not an attack".
"An unidentified object entered our air space and unfortunately as a result it was brought down. It was understood only later that it was a Turkish plane," A Haber quoted Makdissi in a translation of the interview. "There was no hostile act against Turkey whatsoever. It was just an act of defence for our sovereignty."
Turkish President Abdullah Gul conceded the plane may have unintentionally crossed into Syrian airspace, but said such an act was "routine" for jets to unintentionally cross borders for short periods. The government has not described the plane's specific mission.
Gul said his government was still investigating what happened, but "no one should have any doubt that whatever (action) is necessary will be taken".
It was not clear if that action would involve military retaliation, increased sanctions or other steps, including demands for compensation or an apology.
Faruk Celik, the Turkish Labour and Social Security Minister, said his nation would retaliate "either in the diplomatic field or give other types of response".
"Even if we assume that there was a violation of Syria's airspace - though the situation is still not clear - the Syrian response cannot be to bring down the plane," Celik told reporters. "The incident is unacceptable. Turkey cannot endure it in silence."
Germany and Iraq urged the countries to remain calm and not let the unrest in Syria become a wider conflict.
"Our main concern is the spillover of the crisis into neighbourhood countries. No country is immune from this spillover," said Iraq Foreign Minister Hoshyar Zebari.
Turkey has joined the United States and other nations in saying that Syrian President Bashar Assad should step down because of the uprising in his country that has killed thousands of people. Turkey also has set up refugee camps on its border for more than 32,000 Syrians who have fled the fighting.
Turkey said after an April border shooting incident - in which two people in a Turkish refugee camp died - that it would call on its Nato allies to intervene if it felt its security was being threatened.
Turkish Foreign Minister Ahmet Davutoglu met with military officials yesterday to assess what steps to take and to coordinate the search and rescue operation for the two missing pilots and the plane's wreckage, the ministry said. Prime Minister Recep Tayyip Erdogan was expected to discuss the incident with Turkish opposition party leaders today, an aide said.
United Nations Secretary-General Ban Ki-Moon told Turkish Foreign Minister Ahmet Davutoglu that he commended the country for showing restraint in its initial reaction to the incident. He urged both sides to deal with the issue diplomatically, not militarily.