The Israeli military launched an operation on Tuesday to "expose and thwart" tunnels built by the Hezbollah militant group it says stretch from Lebanon into northern Israel.
The tunnels are not operational, the military said, adding that its work to find and neutralise them was taking place inside Israeli territory.
But the operation could send tensions soaring between Israel and its Iranian-backed foe, who have both been preoccupied with other conflicts since their last conflagration more than a decade ago.
"We see Hezbollah's activities as a flagrant and blatant violation of Israeli sovereignty" and of UN resolutions, said Lt. Col Jonathan Conricus, a military spokesman.
"This activity is another example of the negative effects of Iranian entrenchment in the region." The military did not disclose how many tunnels snake into Israeli territory from Lebanon or what tools it was using to deal with the threat, although it said the operation could last for weeks.
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.
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A Lebanese military official told The Associated Press that Lebanese troops and military intelligence agents, along with UN peacekeepers deployed in southern Lebanon, were observing the border on Tuesday.
The official, who spoke on condition of anonymity in line with regulations, had no further details. Israel has been using earthmovers and other large machinery to build a massive wall along its northern border, saying the barrier is needed to protect civilians from Hezbollah attacks.
While the construction has prompted complaints from the Lebanese army, Hezbollah has not responded, an indication it could remain restrained amid the new operation.
Israel, meanwhile, gave no special orders to residents of northern Israel, signaling that it too was not expecting an immediate response from Hezbollah. The existence of the tunnels did not appear to have surprised Israel. Conricus said Hezbollah has been developing an offensive plan against Israel that would "shift the battleground into Israel."
The group would use firepower and ground units and "the surprise component of that plan was supposed to be tunnels that would allow infiltrators into Israel."