A roadside bomb wounded four Israeli soldiers patrolling the Golan Heights on Tuesday, and Israel retaliated with artillery fire on Syrian army positions, the army said.
It was not clear who had planted the bomb in an area where the Syrian military, Lebanese Hezbollah guerrillas and Syrian rebels fighting President Bashar Assad all have a presence.
Violence in Syria has spilled over the Golan frontline in the past but Tuesday's casualties were the worst Israel has suffered in there since the Syrian uprising began three years ago, army spokesman Lieutenant-Colonel Peter Lerner said.
Noticing suspicious movement near the Golan separation fence, the soldiers left their patrol vehicle to inspect it on foot and were hit by an explosion, Lerner said. One was seriously hurt and the others had moderate to light injuries.
Israeli artillery shelled Syrian army positions on the far side of the fence in retaliation, Lerner said.
"We see the Syrian army as responsible, and that is indicated by our response to the attack," he said.
His language suggested Israel was blaming Damascus because it had formal authority over the Syrian-held side of the Golan.
Lerner declined to be drawn on whether Israel knew who specifically had planted the bomb. Two weeks ago, Israel said it foiled a similar attack when its forces shot two Hezbollah men near the Golan fence. Hezbollah has fighters in Syria helping Assad combat a rebellion led by Sunni Islamist insurgents.
In Jerusalem, Israeli Prime Minister Netanyahu said the Syrian-held side of the Golan was "filled" with al Qaeda-linked rebels and Hezbollah guerrillas who, while at each other's throats in Syria, shared deep enmity for the Jewish state.
"This presents a new challenge for the State of Israel," Netanyahu told his Likud faction.
Israel has accused Hezbollah of setting up positions on the other side of the boundary fence. On Friday, an explosive device was detonated against Israeli soldiers patrolling the nearby border with Lebanon, causing no injuries, the army said.
Hezbollah accused Israel of carrying out an air strike on one of its bases on the Lebanon-Syria border last month and vowed to respond. Israel said it would hold the Beirut government responsible if Hezbollah attacked it from Lebanese territory.
Writing by Dan Williams; Editing by Crispian Balmer and Alistair Lyon