An Israeli teenager was killed and more than 20 other people injured Monday by a suspected leftover Hezbollah bomb as they hiked near the Lebanese border, officials said.

The Israeli military, which dispatched helicopters to the scene, said an initial review showed the explosion was likely caused by an unexploded munition from Lebanese militant group Hezbollah. The explosive, which was likely a cluster bomb from a round of fighting three years ago, went off in the hands of a pupil who picked it up, Israeli police said.

Police said that school authorities had received the go-ahead for the outdoor excursion in the region, which Hezbollah has hit in successive rounds of fighting as Israel pounds the Iran-backed group. “I share in the heavy grief of the family of the boy who was killed in the explosion during the trip near the northern border, and I wish a swift and full recovery to all the injured,” Defence Minister Israel Katz posted on social media, adding that the incident was being investigated. Foreign Minister Gideon Saar said the victim was a 15-year-old who had gone on an annual school trip.

In addition to the one dead, more than 20 others were wounded, Northern Command police chief Meir Eliyahu said. The northern city of Haifa took in three teenagers with moderate injuries and a 61-year-old with severe injuries, Rambam hospital spokesman David Ratner said.

The incident injured both students and staff from a religious high school as they were on an outing, the municipality of the northern city of Maalot-Tarshiha said.

Israeli Prime Minister Benjamin Netanyahu scrapped plans and was holding assessments over the “severe incident”, his office said. “The Prime Minister asked to ensure that the injured are being evacuated as quickly as possible to hospitals and is continuing to hold ongoing assessments,” Netanyahu’s office said in a statement.

The incident comes ahead of October 27 elections in which Netanyahu is facing a tough race for a new term.

The opposition is accusing him of not doing enough to ensure Israelis’ security after three years of war, despite recent quiet from Gaza, which Israel has devastated since the October 7, 2023 attacks by Hamas.

Gadi Eisenkot, who is the principal challenger in the elections, said he was “praying together with all Israelis for the swift recovery of the wounded” and wished strength to rescue teams.

Yair Golan, head of the centre-left Democrats party, said three years of periodic fighting with Hezbollah had left Israel’s northern border area with “dangers that cannot be ignored”.

He called for “a thorough and comprehensive inspection of the hiking areas in the north”, including removing unexploded ordnance.

Earlier this year, Hezbollah drew Lebanon into the Middle East war with rocket fire at Israel in solidarity with its backer Iran, which was attacked by Israel and the United States.

Israel responded with air strikes and a ground invasion into Lebanon, where authorities say more than 4,300 people have been killed since March.

Israel and Lebanon agreed to a framework deal that involves the disarmament of Hezbollah, a gradual withdrawal of Israeli forces and the deployment of the Lebanese army to the region.

Violence in the border area has since decreased. Israel last month carried out a massive explosion in Lebanon to destroy a tunnel complex where it said Hezbollah kept weapons and fighters.