Israeli forces used the Beaufort castle, also known as Qalaat al-Chakif, as a base during their previous two-decade occupation of southern Lebanon that ended in 2000.

As Israel seeks to extend its area of control in Lebanon beyond the Litani river, the castle has once again emerged as a strategic point -- and a heritage site at risk of being damaged by war.

- Ancient castle, strategic location -

The castle was originally built by the King of Jerusalem around 1137 and is "one of the best-preserved examples of medieval castles in the Near East" according to UNESCO.

The UN body has now rung the alarm on the safety of southern Lebanon's ancient monuments and landmarks, including Beaufort, as the conflict intensifies.

Perched on a lofty ridge that dominates southern Lebanon and northern Israel, extending into the Israeli-occupied Golan Heights, the stony redoubt has been seized, lost, won again and abandoned by successive armies for nearly nine centuries.

Crusaders and Muslim forces once battled for control of the fortress, which changed hands several times between them.

Centuries later, its elevated position and location about five kilometres from Lebanon's southern border with Israel have turned the castle once again into a strategic position as Israel seeks to widen its ground invasion of Lebanon.

Israeli Prime Minister Benjamin Netanyahu said taking Beaufort was "a dramatic shift" on Sunday as he vowed to push further into Lebanon.

This came days after the Israeli military issued a sweeping evacuation order to areas south of the Zahrani River, north of the Litani and around 40 kilometres (25 miles) from the border.

"This is the first time that Israel is conquering the Beaufort since the withdrawal in 2000," Orna Mizrahi, a senior researcher at Israel's Institute for National Security Studies, told AFP.

Amid threats to northern Israeli communities, "There was a lot of pressure on the government, on the cabinet, to do something more," she said.

"I think that this is one of the main motivations... to go to a deeper military action in Lebanon."

- Civil war history -

During Lebanon's 1975-1990 civil war, Palestinian Liberation Organisation militants used the castle as an observation point in their fight against Israel.

Israel overran the fortress during its 1982 invasion of Lebanon, after a prolonged battle with the Palestinian fighters hidden in the castle's maze of historic underground tunnels.

The castle was damaged by violent bombardment in the process.

Israel used it as one of its main observation posts until its troops withdrew in 2000, particularly for electronic listening, on the edge of the central sector of its so-called security zone.

- Latest strikes -

On Sunday, the invading army's banner was seen by AFP above the castle.

Hezbollah says its fighters are still battling Israeli troops near the fortress, where it says it had no military presence.

Earlier last week, an AFP correspondent saw smoke rising near Beaufort castle after what appeared to be artillery fire.

Lebanon's culture minister had told AFP that Israeli strikes on the country's south were endangering heritage sites and that "several bombs fell" on the fortress.

The region around the Beaufort castle is "the epicentre of the battle being waged for the control of localities" close to it, in the Nabatieh region, Ghassan Salame had told AFP.