Israeli officials have long warned the threat posed by Gaza's Hamas rulers pales in comparison to that of Lebanon's Iran-backed Hezbollah - a heavily-armed mini-army with valuable combat experience and an arsenal of some 150,000 rockets that can reach almost every part of Israel.
"The digging of the cross-border attack tunnels that the IDF has discovered, before the attack tunnels became operational and posed an imminent threat to the safety of Israeli civilians, constitutes a flagrant and severe violation of Israeli sovereignty", the army said in a statement.
In a tweet, Israeli military spokesperson Avichay Adraee said the Lebanese government is responsible for the build-up of the tunnels, saying they are endangering Lebanese citizens.
The area around the Israeli town of Metula has been declared a closed military zone, with the army distributing images of heavy machinery digging into the ground.
Since 2014, an IDF joint-task force of the Intelligence Directorate and the Northern Command has been tackling the problem of Hamas terror tunnels, and in 2015 it began a project to improve the defense of the northern line, including exposing and cutting off the tunnels and constructing a seven-mile (11-kilometer) stone fence.
Netanyahu said he had planned to discuss with Pompeo "steps we are taking together to block the aggression of Iran and its proxies in the north", referring to Syria and Lebanon.
Hezbollah has not responded to the military operation yet.
Israel has launched an operation to destroy tunnels built by Hezbollah across the border from southern Lebanon, the military announced early Tuesday.
Hezbollah is also a powerful political party in Lebanon. Israel has generally refrained from engaging in Syria's civil war, though it has carried out scores of airstrikes against what Israel says were Iranian shipments of advanced weapons bound for Hezbollah.
Conricus said that Hezbollah has been developing an offensive plan against Israel that would "shift the battleground into Israel".
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.
There has been increased attention in Israel in recent days over Israeli officials' concerns regarding Iranian activity in Lebanon.