The Middle East is braced for a potential flare-up in violence after Israeli authorities said a rocket from Lebanon struck a football field in the Israeli-controlled Golan Heights, killing 12 children and teenagers in what the military called the deadliest attack on civilians since October 7.

It raised fears of a broader regional war between Israel and Hezbollah, which in a rare move denied it was responsible for the attack.

The White House National Security Council said it was speaking with Israeli and Lebanese counterparts and working on a diplomatic solution to “end all attacks once and for all” in the border area between Israel and Lebanon.

The Israeli military said it struck a number of targets inside Lebanon overnight into Sunday, though their intensity was similar to months of cross-border fighting between Israel and the Iranian-backed Hezbollah.

Hezbollah said it also carried out strikes. There were no immediate reports of casualties.

Saturday’s attack came at a sensitive time as Israel and Hamas are negotiating a ceasefire proposal to end the nearly 10-month war in Gaza.

The rocket slammed into a football field where dozens of children were playing in the town of Majdal Shams, about seven miles south of Lebanon and next to the Syrian border.

Twelve were killed and 20 others wounded, according to the Israeli military.

The Israeli military said it was investigating why the rocket was not intercepted and questioned whether it would have been possible due to its short journey and short reaction time. A bomb shelter was next to the blackened field, steps away.

Attacks along the Israel-Lebanon border have simmered below the threshold of all-out war since the start of the conflict in Gaza. But the toll and young victims in Saturday’s attack could push Israel to respond more severely.

Israeli Prime Minister Benjamin Netanyahu was weighing options on Sunday after hurrying home from the US and warning that Hezbollah “will pay a heavy price for this attack, one that it has not paid so far”. The security cabinet authorised him and defence minister Yoav Gallant to decide how and when to respond.

US secretary of state Antony Blinken said on Sunday that “every indication” showed the rocket came from Hezbollah.

The Israeli military’s chief of staff, Lt Gen Herzi Halevi, said a Falaq rocket with a 53kg warhead that belonged to Hezbollah was fired.

Hezbollah began firing rockets at Israel the day after Hamas’ attack on October 7. Israel has responded by targeting what it says is Hezbollah’s military infrastructure with air strikes and drones.

Most attacks have been confined to border areas, though Israel has assassinated Hezbollah and Hamas leadership further north in Lebanon. Tens of thousands of people along the border have been evacuated.

Since early October, Israeli air strikes in Lebanon have killed more than 500 people, mostly Hezbollah members but also around 90 civilians. On the Israeli side, 22 soldiers and 24 civilians have been killed.

Hezbollah has far superior firepower to Hamas. Igniting a war in Israel’s north while it is engaged in Gaza would overburden the military, Barak Ben-Zur, a researcher at the International Institute of Counter-Terrorism, told journalists.

In Lebanon, some prepared for more fire from Israel. Lebanon’s national airline announced it had postponed the Beirut arrival of seven flights until Monday morning, without saying why. Caretaker Prime Minister Najib Mikati had urgent calls with diplomats and politicians, his office said.

Any conflict could bring in Iran, which warned Israel that a strong reaction to the Golan Heights strike would lead to “unprecedented consequences”.

Iran and Israel’s shadow war burst into the open in April, when Iran launched 300 missiles and drones at Israel, most of them intercepted, in response to the killing of an Iranian general.

The United Nations secretary-general called for maximum restraint by all parties.

Meanwhile, an Egyptian official said the attack in the Golan Heights could give urgency to negotiations to reach a ceasefire deal in Gaza.

“Both fronts are connected,” he said. “A ceasefire in Gaza will lead to a ceasefire with Hezbollah.”

In a statement, the Egyptian foreign ministry called on all influential international players to “intervene immediately to spare the peoples of the region further disastrous consequences of the expansion of the conflict”.

Officials from the United States, Egypt and Qatar were meeting on Sunday with Israeli officials in Rome in the latest push for a deal.

The head of Israel’s Mossad spy agency, David Barnea, later returned home and negotiations will continue in the coming days, Mr Netanyahu’s office said.