Israel released video footage of digging and pile-driving equipment at work in unidentified locations, carrying out what it said were "tactical preparations to expose Hezbollah's offensive cross-border tunnel project".
There was no immediate comment from Hezbollah and its Al-Manar TV did not make any mention of the Israeli operation.
Israeli drilling equipment is seen next to the border with Lebanon, near the Lebanese village of Kfar Kila, seen from the Israel's side December 4, 2018.
The IDF said that the tunnel was equipped with electrical and communication lines and would have taken some two years to build, likely due to the harsh terrain along the border.
The surprise announcement came hours after Israeli Prime Minister Benjamin Netanyahu met with US Secretary of State Mike Pompeo in Brussels over regional dangers, with both having repeatedly warned over the activities of Iran, Israel's main enemy.
"Whoever tries to harm Israel will pay a heavy price", Netanyahu said in a statement after returning to Israel early in the morning. On its Twitter account, the Lebanese army said its forces were "closely" following developments and were "totally prepared to confront any sudden occurrence". An Israeli military source said the operation might take weeks to complete.
The military did not disclose how many tunnels snake into Israeli territory from Lebanon or how long the operation would last.
Israel launches operation to thwart Hizbollah tunnels from Lebanon
The operation was launched before dawn on Tuesday, with the army declaring some areas adjacent to the Lebanon border a closed military zone and deploying additional forces to the north. Israel has spent years attempting to tackle a network of tunnels from the Gaza Strip into Israel and has used a variety of methods to destroy tunnels and prevent them from being rebuilt. The military said it was assessing the situation to gauge Hezbollah's possible reaction.
The Lebanese army is ready for any developments on the border with Israel making efforts together with the United Nations forces to maintain stability in the area, the army said in a statement.
It began when Hezbollah militants launched a raid into Israel, killing eight soldiers and kidnapping two others, who also died.
Israel has long voiced concern about Iran's growing military presence along its northern border, warning of an Iranian corridor that could assist in the transfer of weapons to Hezbollah.
"The Hezbollah terror organization, which is responsible for digging the cross-border attack tunnels, continues activity that is supported and funded by Iran, to establish its terror infrastructure against Israeli civilians".
Since the start of that war, more than 200 strikes were carried out by Israel according to its military.
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The former Porto, Chelsea, Inter Milan and Real Madrid coach has spent £360m since he replaced Louis van Gaal at United in 2016. United were unable to go one better and get the win, however, and they'd drop two valuable points against Southampton .