High up on the dry brush hills that rise steadily from the Israeli border, the UN watchtower surveys southern Lebanon and the plains below as far as the eye can see.

But no more than 100 metres from the blast walls below the tower, a rusty metal trapdoor swings open to reveal a tunnel cut deep into the rock.

This, the Israeli military says, is the entrance to a Hezbollah attack position.

It is one of two within a stone’s throw of a UN base where international soldiers have for years rotated in and out to prevent another war breaking out.