Israel’s prime minister, Benjamin Netanyahu, vowed the Lebanese militant group Hezbollah would “pay a hefty price” after a strike on a football pitch in a remote town in the occupied Golan Heights killed11 young people and a daylong barrage of rocket fire from Lebanon targeted Israeli territory.

Israel’s Magen David Adom emergency services said “large numbers” of ambulances were dispatched to the scene to treat the casualties, all aged between 10 and 20 years old. Video and imagery showed young casualties strewn across the grass, some wearing sports shirts.

The attack struck the predominantly Druze town of Majdal Shams in the mountainous Golan Heights, close to the border with Syria. Israel has occupied the area since 1967, annexing it in 1981.

In a statement from Netanyahu’s office, the Israeli leader said he told the head of Israel’s Druze community that “Hezbollah will pay a heavy price, the kind it has thus far not paid.”

A spokesperson for the UN peacekeeping force operating in southern Lebanon told Reuters that it had made contact with the Lebanese and Israeli authorities “to understand the details of the Majdal Shams incident and to maintain calm”.

Hezbollah and other militias based in Lebanon have repeatedly struck Israeli territory in retaliation for Israel’s assault on Gaza, while Israeli airstrikes have targeted towns and cities deep into Lebanon. Lebanese officials and rights groups have claimed Israel has used white phosphorus in southern Lebanon.

Almost 200,000 people are displaced on either side of the blue line that separates the two nations, prompting growing anger within Israel amid demands the government prevent further attacks.

“The Hezbollah attack today crossed all red lines, and the response will be accordingly. We are approaching the moment of an all-out war against Hezbollah and Lebanon,” Israel’s foreign minister, Israel Katz, told Axios.

The strike marked a grave escalation after a volley of rocket fire from southern Lebanon directed at several towns in the occupied Golan Heights, the majority of which were claimed by Hezbollah.

However, Mohammad Afif, a senior Hezbollah official, denied responsibility for the strike that hit Majdal Shams, speaking to Reuters. In a statement, the militant group said it had “absolutely nothing to do with the incident”, accusing hostile media outlets of “false allegations”.

The Lebanese government condemned the attack, amid fears of an escalating regional conflict. “Targeting civilians is a flagrant violation of international law and goes against the principles of humanity,” it said in a statement, demanding “an immediate cessation of hostilities”.

Netanyahu’s office said he would fly home early from a visit to the US, where he met the president, Joe Biden, vice-president and candidate, Kamala Harris, and former president Donald Trump.

“Immediately upon learning of the disaster, prime minister Benjamin Netanyahu directed that his return to Israel be brought forward as quickly as possible,” they said.

The Israel Defense Forces as well as Israeli officials attributed the Majdal Shams strike to Hezbollah.

A spokesperson for the White House’s national security council, which has long attempted to help broker a ceasefire in Gaza, also condemned the strike. US negotiators have been working for months to cool tensions on Israel’s northern border.

“Israel continues to face severe threats to its security, as the world saw today, and the United States will continue to support efforts to end these terrible attacks along the blue line, which must be a top priority,” they said.

Bezalel Smotrich, the far-right finance minister and a member of Israel’s security cabinet, said: “For the death of small children, [Hezbollah secretary general Hassan] Nasrallah should pay with his head. Lebanon as a whole has to pay the price … It’s time for action!”

Isaac Herzog, the president, said: “The heart breaks in the face of the shocking and terrible disaster in the Druze village of Majdal Shams.” He accused Hezbollah of attacking “children whose only sin was going out to play football. And never returned.”

The barrage of rocket fire followed a strike by the Israeli air force on the town of Kafr Kila in southern Lebanon, close to the line that demarcates Lebanese and Israeli territory. The strike killed four, including several members of Hezbollah, according to Reuters and a statement from the group.

Israeli forces also said they intercepted a drone from Lebanon targeting an offshore gas rig, while Lebanon’s state-run news agency said an Israeli drone attack struck the border town of Odaisseh sparking a fire.

Artillery on the town of Meiss el-Jabal also started fires in southern Lebanon, while Israeli fighter jets broke the sound barrier over the southern city of Tyre.