Twenty-four Israeli soldiers were killed and 11 seriously injured overnight as Israel tripled its forces and pushed forward to capture more territory before an imminent ceasefire.
The casualties, confirmed by the Israeli army on Sunday, were the heaviest in one day for the Jewish state in its 32-day offensive against the Hezbollah.
The Israeli military claimed to have killed at least 50 guerillas.
Five soldiers were killed on Saturday night when the Hezbollah militants shot down an Israeli helicopter over south Lebanon.
The helicopter had crashed in enemy territory, the military said. The bodies of the soldiers have not yet been recovered.
Hezbollah's al-Manar TV said that the helicopter was brought down by a new missile, the "Wa'ad" (Arabic for 'Promise'), in the guerrilla arsenal.
This was the fourth helicopter to crash since fighting began at the Israel-Lebanon border on July 12.
In other clashes with Hezbollah operatives on Saturday, 19 more soldiers were killed and 11 seriously wounded, making it the bloodiest day for Israel in the new flare up.
Military operations in Lebanon continued to expand, with armoured, infantry and engineering forces extending deep into Lebanon to cover the entire region between the Israeli border and the Litani river before a ceasefire was declared.
Israel has signalled its intention to approve the UNSC ceasefire plan on Sunday in its weekly cabinet meeting, and UN Secretary General, Kofi Annan, has announced that the fighting would end 8 am local time (1030IST) on Monday.