Turkish forces have shot down a Syrian military jet they say was violating their airspace despite warnings.
Turkey's Prime Minister Recep Tayyip Erdogan warned such action by Syria merited a "heavy response".
But Syria accused Turkey of "blatant aggression", saying the plane had been over Syrian territory at the time.
The incident reportedly occurred in an area where Syrian rebels and government forces have been fighting for control of a border crossing.
Turkey and Syria - once allies - share more than 500 miles (800km) of border.
Turkey has broadly sided with the rebels in Syria's war since October 2011.
Turkish forces launched artillery strikes on Syrian targets in late 2012 after the Syrians shot down a Turkish jet.
However, the BBC's James Reynolds in Istanbul says neither side is interested in a direct, sustained conflict.
Speaking at a rally of supporters, Mr Erdogan congratulated the air force on its actions on Sunday.
"A Syrian plane violated our airspace. Our F-16s took off and hit this plane. Why? because if you violate my airspace, our slap after this will be hard," he said.
A Syrian military source, quoted by state television, said Turkish air defences had shot down a Syrian jet as it attacked rebels on Syrian territory - an act of "blatant aggression".
The Syrian Observatory for Human Rights - a UK-based activist group - said initial reports from the area suggested the plane came down on the Syrian side of the border.
"Turkish air defences targeted a Syrian fighter-bomber as it struck areas of the northern province of Latakia. The plane caught fire and crashed in Syrian territory," the Observatory said.
According to one report, the plane's pilot was able to eject.
In a media statement on the incident, the Turkish General Staff said two Syrian military jets were involved, and that they had been "warned four times that they were approaching Turkish airspace".
One of the Syrian MiGs turned back, it said.
"But the second Syrian jet entered Turkish airspace in the Camli Hill Border Outpost area in Yayladag, Hatay at 1313 (local time) in spite of the warnings. It then turned westwards and continued flying in our airspace for a distance of 1.5km," the statement said.
"At this point one of two Turkish F-16s flying Combat Air Patrol in the region fired a missile at the Syrian jet in accordance with the rules of engagement at 1314. Hit, the Syrian jet fell in the vicinity of Kesab on Syrian soil 1,200m south of the border."
In September last year, Turkey said it had shot down a Syrian helicopter close to its border. Deputy Prime Minister Bulent Arinc said the aircraft was engaged by fighter jets after violating Turkish air space.
A Turkish fighter jet was shot down by Syria over the Mediterranean in June 2012, after Syrian forces said it had entered its airspace.
Hundreds of thousands of people have fled Syria for Turkey to escape the three-year uprising against President Assad.
More than 100,000 people have been killed since the Syrian conflict began.
According to UN figures, 6.5 million Syrians have been displaced by the civil war, and 2.5 million are registered as refugees. Lebanon has taken the highest number of refugees, followed by Jordan and Turkey.