Israel said it attacked hundreds of Hezbollah targets on Monday in airstrikes which Lebanese authorities reported killed at least 274 people in Lebanon's deadliest day in decades, ramping up nearly a year of conflict between Israel and its Iran-backed enemy.

After some of the heaviest cross-border exchanges of fire since the hostilities flared, Israel warned people in Lebanon to evacuate areas where it said the armed movement was storing weapons.

After almost a year of war against Hamas in Gaza on its southern border, Israel is shifting its focus to its northern frontier, from where Hezbollah has been firing rockets into Israel in support of its ally Hamas.

Israel's military on Monday struck Hezbollah in Lebanon's south, eastern Bekaa valley and northern region near Syria in its most widespread strikes.

Lebanon's health minister said 274 people were killed, including 21 children and 39 women, and 1,024 were wounded in Israel's strikes in what one Lebanese official said was the highest death toll since the end of the 1975-1990 civil war.

Prime Minister Benjamin Netanyahu said Israel faced "complicated days" as it stepped up attacks in southern Lebanon and called on Israelis to stay united as the campaign unfolded.

"I promised that we would change the security balance, the balance of power in the north – that is exactly what we are doing," he said in a message following a situational assessment at military headquarters in Tel Aviv.

Earlier, Israel's Defence Minister Yoav Gallant said the actions would continue until "we achieve our goal to return the northern residents safely to their homes," setting the stage for a long conflict as Hezbollah has vowed to fight on until there is a ceasefire in Gaza.

The Israeli military said it had struck about 800 targets connected to Hezbollah in southern Lebanon and the area of the Bekaa valley. "Among the targets struck were buildings where Hezbollah hid rockets, missiles, launchers, UAVs and additional terrorist infrastructure," the military said in a statement. (Reuters)