Israel's Foreign Ministry said Sunday that Hezbollah had "crossed all red lines" with a rocket strike blamed on the Lebanese armed group that killed 12 youngsters in the annexed Golan Heights.

"Saturday's massacre constitutes the crossing of all red lines by Hezbollah. This is not an army fighting another army, rather it is a terrorist organization deliberately shooting at civilians," the ministry said in a statement.

Iran-backed Hezbollah, which on Saturday claimed multiple attacks on Israeli military positions following a deadly raid on southern Lebanon, has denied responsibility for the strike that hit the Golan Heights town of Majdal Shams.

The foreign ministry said "an Iranian rocket" caused the deaths of "our boys and girls."

"Hezbollah is the only terror organization which has those (rockets) in its arsenal," it said.

"Israel will exercise its right and duty to act in self-defense and will respond to the massacre."

The military said the rocket attack killed 12 people aged between 10 and 20 years when they were hit on a football field in Majdal Shams, a Druze town where many residents have not accepted Israeli nationality since the Golan Heights was seized from Syria in 1967.

Prime Minister Benjamin Netanyahu, who was returning early from a visit to the United States, vowed that "Israel will not let this murderous attack go unanswered."

He was due to head a security committee meeting later on Sunday, his office said.

Also on Sunday, Iran warned Israel that any new military "adventures" in Lebanon could lead to "unforeseen consequences."

"Any ignorant action of the Zionist regime can lead to the broadening of the scope of instability, insecurity and war in the region," said foreign ministry spokesman Nasser Kanani.

He added that Israel will be responsible for "the unforeseen consequences and reactions to such stupid behavior."

(With input from AFP)