But on Monday, a senior Israeli official said Israel is starting to question that assessment. "We thought it was spillover, but today we're not sure," said the official, speaking on condition of anonymity because the issue is still being debated among intelligence commanders.
Syria's civil war also shook the country's northern neighbor, Turkey, on Monday, after a Syrian fighter jet bombed a rebel-held area near the frontier, killing at least six people, a Turkish official said on condition of anonymity because he was not authorized to brief the media.
Israeli officials have long feared that the embattled Assad might try to draw Israel into the fighting in an act of desperation.
In a statement, the military said Israeli tanks targeted the "source of fire" in Syria after the mortar shell landed in an open area of the Golan Heights. It confirmed "direct hits" on the targets.
Israeli military officials, speaking on condition of anonymity under army guidelines, said an armored 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, a strategic plateau, from Syria in the 1967 Mideast war and subsequently 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.
Israel has little love for Assad, who has provided refuge and support to Israel's bitterest enemies through the years. But the Syrian leader — and his father before him — have kept the frontier quiet for nearly four decades, providing a rare source of stability in the volatile region.
Open hostilities between Israel and Syria could have wide-ranging consequences, dragging in Lebanon's Hezbollah guerrilla group and perhaps Hamas militants in the Gaza Strip on Israel's southern flank.