A mortar attack from inside Syria has killed a 15-year-old Israeli in the Golan Heights, Israeli officials said.

The boy was killed as a car carrying civilian contractors working for the Israeli defence ministry exploded in the Tel Hazaka area near the demarcation line with Syria on Sunday, a senior military source told Al Jazeera.

The disputed border region has seen sporadic exchanges of fire since the war in Syria began three years ago, but this was the first fatality on Israel's side of the frontier.

"The defence forces say it appeared this was a targeted attack, that it didn't look like an incident," Al Jazeera's Jane Ferguson, reporting from Jerusalem, said.

Rebels are fighting the Syrian regime near the border and stray mortar rounds have fallen in the Israeli-occupied Golan Heights on several occasions.

Israel has started building a fence on the frontier, which the contractors were working on, according to the military source.

Initial reports said a civilian contractor was killed in the explosion but it later emerged that the victim was a youth who accompanied him. Two other people were reportedly wounded.

Retalitary shelling

The Israeli army responded with tank fire targeting military posts in Syria after the incident.

"This mortar fire was unprovoked, and we have the right to respond," Mark Regev, spokesman for Prime Minister Benjamin Netanyahu, told Al Jazeera.

Israel seized the Golan Heights from Syria during the 1967 Six Day war, and most of the Syrian residents fled during the conflict.

Since the two countries signed an armistice in 1974, after a thwarted Syrian attempt to retake the land, a UN observer force has been deployed to the plateau.

Israel unilaterally annexed the territory in 1981, in a move that has not been internationally recognised.

A roadside bomb in the area wounded four Israeli troops in March.

Israeli Defence Minister Moshe Yaalon said at the time that he held the regime of Syrian President Bashar al-Assad responsible for the incident and Israel bombed Syrian military targets in response.