(Adds Israel says 15-year-old killed in blast)
JERUSALEM, June 22 (Reuters) - An attack from inside Syria on Sunday killed a 15-year-old on the Israeli-occupied Golan Heights, the first fatality on Israel's side of the frontier since the Syrian civil war began, the military said.
Israeli tanks fired at Syrian army positions in response to what an Israeli military spokesman described as an intentional attack.
Security officials initially said a civilian contractor for Israel's Defence Ministry was killed in an explosion. But they later said that a youth, aged 15, who accompanied him, had died and that two other people were wounded.
A military spokesman said it was not yet clear whether a roadside bomb or an artillery shell or mortar round, fired from Syria across the frontier fence on the Golan, had struck the water tanker in which the group had been travelling.
"This is the most substantial event that we have had on the border with Syria since the beginning of the (Syrian civil) war," the spokesman said.
The youth was not immediately identified and it was unclear whether he was an Israeli or belonged to the Druze community that lives on the Golan Heights, in territory captured by Israel from Syria in a 1967 war and annexed in a move that is not internationally recognised.
Shelling from the Syrian civil war has occasionally spilled over onto the Golan, including what Israel has said were deliberate attacks on its troops.
While the Syrian army has a presence in the Golan, some areas are controlled by rebels fighting to topple President Bashar al-Assad, including al Qaeda-inspired militants hostile to the Jewish state.
Israel says Hezbollah guerrillas from Lebanon are also operating, on Assad's behalf, on the Golan. Israeli officials have voiced concern that Israel will increasingly become a target during and after the Syrian conflict.
Last March, four Israeli soldiers were wounded in a roadside bombing along the Golan frontier. Israel responded to that incident by launching air strikes against Syrian military sites.
(Reporting by Ari Rabinovitch and Dan Williams, Editing by Jeffrey Heller)