Israel’s air force launched a rare airstrike on a military site inside Syria, the Syrian government and US officials said on Wednesday, adding a potentially flammable new element to regional tensions already heightened by Syria’s civil war. The strike appeared to be the latest salvo in Israel’s long-running effort to disrupt the Lebanese militant group Hezbollah’s quest to build an arsenal capable of defending against Israel’s air force and spreading destruction inside the Jewish state.

US officials said the target was a convoy of trucks that Israel believed contained anti-aircraft weapons bound for Hezbollah in Lebanon.

Regional officials said the shipment included sophisticated Russian-made SA-17 anti-aircraft missiles, which if acquired by Hezbollah, would be “game-changing,” enabling the militants to shoot down Israeli jets, helicopters and surveillance drones.

Syria threatened Thursday to retaliate for the Israeli airstrike, and its ally Iran said there will be repercussions for the Jewish state over the attack.

Syrian ambassador to Lebanon Ali Abdul-Karim Ali said Damascus “has the option and the capacity to surprise in retaliation.”

Hezbollah condemned the attack as “barbaric aggression” and Syrian ally Russia said it appeared to be an unprovoked attack on a sovereign nation.