Israeli jets knocked out TV and phone masts in the east and north of Lebanon, while ground troops carried out operations in two southern villages.
Israel has warned residents in 10 southern villages to leave by 1800 local time (1500 GMT), and thousands of civilians have been fleeing north.
However, Israel has insisted it has no plans for a large-scale invasion.
UN humanitarian chief Jan Egeland is due to arrive in Beirut to assess the crisis on the ground.
Mr Egeland said half a million people needed assistance – and the number was likely to grow.
He is expected to call on Israel to guarantee safe passage by land, sea and air for humanitarian supplies.