The warning called into further question the fragile ceasefire that took effect on April 17, but which has done little to halt the fighting between Israel and Iran-backed Hezbollah.

"In light of the repeated violations of the ceasefire agreement by the terrorist organisation Hezbollah, the IDF will act against it with great force," the military's Arabic-language spokesman Avichay Adraee posted on X.

"We advise the residents of southern Lebanon to evacuate to the north of the Zahrani River, as all areas south of the river are considered combat zones."

The message comes after Israel vowed this week to intensify operations in Lebanon and said it was expanding ground operations in the south.

Under the terms of the ceasefire published by Washington, Israel reserves the right to act against "planned, imminent or ongoing attacks".

Israel's army chief Lieutenant Colonel Eyal Zamir said on Wednesday that "we are intensifying our operations in order to strike ever more severe blows to the Hezbollah organisation".

"This mission is being carried out methodically, on all fronts -- both in the air and on the ground -- with responsibility and determination, in the face of an enemy that is weakened and severely tested," he said during an air force ceremony, according to a video published by the military.

"Hezbollah is deploying a wide range of threats against us, including the use of drones," he added, vowing to step up efforts aimed at "imposing a heavy cost on the enemy".