Israeli troops have captured the 900-year-old Beaufort Castle and its strategic ridge in southern Lebanon in a major advance against Hezbollah that took them beyond the Litani River – their deepest incursion into the country in more than 26 years.

After days of intense fighting and airstrikes in nearby villages, the Israeli defence minister, Israel Katz, said the military had captured the fortress, also known as Qalaat al-Shakif, which the Israel Defense Forces used as a base during their previous occupation of southern Lebanon between 1982 and 2000.

Images published by Katz and footage verified by Agence France-Presse showed Israeli and Golani Brigade flags flying over Beaufort Castle as shelling echoed across the surrounding hills and plumes of smoke rose from the area. Perched on a commanding hilltop, the medieval fortress overlooks much of southern Lebanon, giving it significant strategic importance.

In a statement the IDF said it had “launched an operation in the Beaufort Ridge and Wadi al-Saluki area of southern Lebanon to dismantle Hezbollah infrastructure and expand its control of the area”.

The military said its forces had crossed the Litani River and seized positions from which Hezbollah had directed attacks and launched rockets into Israel.

Stalled negotiations between the US and Iran have left a vacuum in which Israel has stepped up its operations in Lebanon. Observers say Israeli officials and military commanders wanted to inflict as much damage as possible on Hezbollah before any deal between Tehran and Washington imposed new limits on or stopped the current offensive.

Israeli forces appear to be positioning themselves for a potential encirclement of Nabatieh, a city that serves as an economic centre and a cultural heartland for southern Lebanon. Control of the surrounding hills would provide commanding views over large parts of southern Lebanon and the western Bekaa valley, offering a significant tactical advantage.

A truce to halt the fighting between Israel and Iran-backed Hezbollah officially began on 17 April but has never been observed. Israel and Hezbollah accuse each other daily of violating the ceasefire and justify their attacks by the other’s alleged breaches.

For many Lebanese, Nabatieh carries a significance that extends beyond its strategic value. Long regarded as a symbol of resistance, the city has repeatedly found itself on the frontline of Israeli military campaigns and remains deeply embedded in the political and historical memory of southern Lebanon.

In recent days Israeli forces have moved past the towns of Zawtar al-Sharqiyah and Mayfadoun and are now approaching Choukine, where local people were ordered to evacuate on Saturday amid fears of further military operations.

On Saturday Lebanon’s prime minister, Nawaf Salam, accused Israel of “pursuing a scorched-earth policy and collective punishment” by “destroying towns and villages, and forcing their inhabitants into exile” in the south of the country. Salam warned the country was facing a “dangerous” escalation and called for “a swift and real ceasefire”.

The actions would bring “neither security nor stability” to Israel, he said.

Salam defended his government’s engagement with its southern neighbour, after military delegations from both countries held security talks in Washington on Friday, with more US-brokered negotiations planned next week.

Salam said the outcome of the negotiations was “not guaranteed”, but called them “the least costly path for our country and our people”.

Reuters reported the Israeli military as saying one of its soldiers had been killed in combat in southern Lebanon.

Earlier on Saturday, Hezbollah said it had targeted the air traffic control unit at the Meron base in northern Israel, a strategic surveillance and command facility near the Lebanese border. The group also claimed responsibility for rocket fire towards Kiryat Shmona, one of the Israeli communities most exposed to the conflict.

Videos circulating on social media appeared to show beachgoers in northern Israel running for shelter as Hezbollah rockets were launched towards the area, according to local media. The barrage was the first fired from Lebanon towards the coastal city of Nahariya in three weeks.

The Lebanese health ministry says Israeli attacks have killed more than 3,371 people since 2 March, when Hezbollah drew Lebanon into the Middle East war in support of its backer Iran. Hezbollah said it attacked Israel in retaliation for the death of Iran’s supreme leader in US-Israeli strikes when the war erupted on 28 February. Iran has insisted that any agreement to end the wider Middle East war must also cover Lebanon.

With Agence France-Presse and Reuters