An Israeli tank scored a direct hit on a Syrian artillery unit yesterday after a mortar shell fired from Syria landed in the Golan Heights.
The first direct confrontations between the countries since the Syrian uprising began are sharpening fears that Israel could be drawn into the civil war next door, something it has steadfastly tried to avoid.
However, Israel has grown increasingly worried after a series of stray mortar shells struck territory in the Israeli-controlled Golan Heights.
Israeli military officials say they believe the mortar fire is spillover from intense fighting near the frontier between troops loyal to the Syrian president Bashar Al Assad and rebel forces trying to oust him, and not an overt attempt to strike Israeli targets.
Yesterday, however, a senior Israeli official said Israel was starting to question that assessment. "We thought it was spillover, but today we're not sure," he said.
The Israeli army warned that any further fire from Syria towards the Israeli-occupied sector of the strategic plateau would not be ignored.
"Fire emanating from Syria into Israel will not be tolerated and shall be responded to with severity," a statement said. Israel has filed a complaint with United Nations observers monitoring the ceasefire line.
Syria's civil war also shook the country's northern neighbour, Turkey, yesterday, after a Syrian fighter jet bombed a rebel-held area near the frontier, killing at least six people, a Turkish official said.
Israeli officials have long feared that the embattled Mr Assad might try to draw Israel into the fighting in an act of desperation.
In yesterday's exchange, Israeli military officials said an armoured vehicle carrying "Syrian mobile artillery" was hit. There was no immediate word on casualties on the Syrian side, but Israeli officials said the vehicle was believed to belong to the Syrian government.
A number of mortar shells have landed in the Golan over the past week, and early this month Syrian tanks accidentally crossed into a buffer zone along the frontier of the Golan for the first time in nearly 40 years.
Israel captured the Golan from Syria in the 1967 war and later annexed it.
After weeks of standing still, Israel responded for the first time on Sunday, firing what it called a "warning shot" into Syria after another mortar shell strayed across the frontier and landed near an Israeli military post. Israel also warned of a tougher response if the attacks persisted.
While Israel appeared eager to calm the situation, its response was a potent reminder of how easily the Syrian civil war - already spilling across borders with Turkey, Lebanon and Jordan - could explode into a wider regional conflagration.
* With reporting by the Associated Press and Agence France-Presse