A boy walks past bicycles left next to the area that was hit by a rocket, killing multiple children and teenagers, on a soccer field in the Druze town of Majdal Shams, in the Israeli-annexed Golan Heights, on Saturday. AP

Israel vowed on Sunday to "hit the enemy hard" after rocket fire from Lebanon killed 12 young people in the Israeli-annexed Golan Heights and fanned fears that the war in Gaza will spread.

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

UN Secretary-General Antonio Guterres condemned Saturday's rocket fire and called on all parties to "exercise maximum restraint".

Israel's army called it "the deadliest attack on Israeli civilians" since the October 7 attack that began the current fighting in Gaza and triggered regular exchanges of fire across the Lebanese border.

Israel blamed Lebanon's Hezbollah movement for firing a Falaq-1 Iranian rocket but the Iran-backed group -- which has regularly targeted Israeli military positions -- said it had "no connection" to the incident.

UN Secretary-General Antonio Guterres attends a meeting. File photo

It said, however, that it had fired one such rocket on Saturday towards an Israeli military target in the Golan.

The rocket fire in Majdal Shams, whose population are Arabic-speaking Druze, prompted Israeli Prime Minister Benjamin Netanyahu to return early from the United States.

Upon arrival he went immediately into a security cabinet meeting, his office said.

"Hezbollah will pay a heavy price" for the attack, "a price it has not paid before", he said.

After the meeting, his office said: "The members of the cabinet authorised the prime minister and the defence minister to decide on the manner and timing of the response against the Hezbollah terrorist organisation." It offered no further details.

Defence Minister Yoav Gallant earlier Sunday visited the scene of the attack and vowed Israel would "hit the enemy hard".

Israel's foreign ministry said Hezbollah had "crossed all red lines".

Agence France-Presse