The capture of Beaufort castle near the city of Nabatiyeh came after days of intense fighting and airstrikes in nearby villages where Israeli troops fought Hezbollah members in the rugged area.
The capture of the castle marks a major gain for Israel since the latest Israel-Hezbollah war began in early March and as the two countries that have been in a state of war since Israel was created in 1948 hold direct talks in Washington.
The Israeli army's Arabic spokesman Avichay Adraee posted a photograph on X showing Israeli troops walking outside the castle.
Lebanon's state-run National News Agency earlier reported Israeli air strikes and artillery shelling near the Beaufort castle that is about 15km from the Israeli border and overlooks wide parts of southern Lebanon.
The castle was held by Israeli troops for 18 years until they withdrew from Lebanon in May 2000.
Israeli troops have been advancing for days in villages close to the castle, including Yohmor and Zawtar al-Sharqieh near the city of Nabatieh after they crossed the strategic Litani River, which the Israeli military has used as a de-facto boundary.
Large areas to the south are under Israeli military control, despite an April 17 US-brokered ceasefire.
NNA reported air strikes on different parts of southern Lebanon including in the village of Ansar that killed three people.
A drone strike on a road linking the village of Ebba with Nabatieh wounded two Lebanese soldiers, the army said in a statement.
Hezbollah, meanwhile, said its fighters fired rockets at northern Israel's largest city - Kiryat Shmona - on the border with Lebanon.
The group said its attack was in retaliation for air strikes that killed civilians in Lebanon.
Hezbollah later said it also fired rockets toward the northern city of Safed.
with DPA
