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BEIRUT (AP) — Syrian rebels fought with the gunmen from the Hezbollah militia in a deadly clash on Lebanese soil, a security official and local media said Sunday, in the latest sign Syria’s civil war is spilling over the country’s borders.

It was the worst clash on Lebanese territory between the two sides since the outbreak of the Syria conflict more than two years ago. The violence highlighted the growing risk the fighting in Syria poses to fragile Lebanon, whose volatile sectarian makeup mirrors that of its neighbor.

Hezbollah and Syria’s rebels fight on opposite sides inside Syria, and tensions between them have risen sharply since the Lebanese militia stepped up its armed support for President Bashar Assad’s regime last month.

Rebel fighters have threatened to attack Hezbollah bases in Lebanon, and on Saturday 18 rockets and mortar rounds hit Lebanon’s eastern Baalbek region, a Hezbollah stronghold.

In the night from Saturday to Sunday, Hezbollah apparently encircled and ambushed a group of Syrian rebels and allied Lebanese fighters whom they suspected of rocketing Baalbek a day earlier, the Lebanese security official said. He spoke on condition of anonymity in line with regulations and because details were not yet clear.

He said a Hezbollah fighter and several rebels were killed in the clashes in a remote area between Baalbek and the Syrian border.

The Lebanese TV station Al-Mayadeen, seen as sympathetic to the Syrian regime, quoted Lebanese security officials as saying 17 fighters from Jabhat al-Nusra, a rebel group linked to the global al-Qaida terror network, were killed in the fighting.