"The neutralisation and destruction phase of the cross-border attack tunnels... has begun," the army said in a statement.

Since launching an operation to cut off the tunnels on December 4, the army says it has located four underground passageways infiltrating Israeli territory.

It issued a warning to Hezbollah, the Lebanese Shiite militant group with which Israel fought a devastating war in 2006.

"The IDF (Israel Defense Forces) continues to operate... with reinforced troops, and is prepared with a wide variety of enhanced capabilities for any developments, should they occur," the army said.

Israel alleges Hezbollah had planned to use the tunnels to kidnap or kill its civilians or soldiers, and to seize a slice of Israeli territory in the event of any hostilities.

The army said it would "continue to operate to neutralise Hezbollah's military infrastructure and to thwart" its any threats against Israel and its civilians.

A month-long war in 2006 between Israel and Hezbollah killed more than 1,200 Lebanese, mostly civilians, and more than 160 Israelis, mostly soldiers.

The Israeli army said the operation to locate and destroy the tunnels was defensive, and that it would involve different techniques.

"This phase will... ensure that the attack tunnels cannot be used by the Hezbollah terror organisation and will prevent it from implementing its plans," said the statement.

The army advised Israeli authorities and UNIFIL, the United Nations peacekeeping mission in Lebanon, to notify its Lebanese counterpart to take measures to protect civilians across the border.

It said it held the Lebanese government "accountable for digging the attack tunnels".

"The IDF warns against approaching the openings of the tunnels or staying close to them on the Lebanese side," said the statement.