Families packed into cars and pickup trucks with their possessions and roads to the north were jammed after Israeli planes dropped leaflets warning residents of south Lebanon to flee for safety beyond the Litani river, about 20km from the border.
An estimated 300,000 Lebanese normally live south of the Litani, the majority of whom are Shia Muslims.
There was no word on how many have already fled the bombing and fighting of the past few days. Air raids have wrecked many roads and bridges in the region.
Hisham Hassan, a spokesman for the international committee of the Red Cross said: "The siege on Lebanon is not letting humanitarian aid in. The south is isolated."
Lebanon will fight
But Elias al-Murr, the Lebanese defence minister, said on Friday that the Lebanese army would fight any Israeli invasion.
"Our constitutional duty is to defend Lebanon as a Lebanese army. This is our role. Any Lebanese citizens, Christians or Muslims, who want to defend their land are welcomed."
But al-Murr said the Lebanese army would not be working directly with Hezbollah.
"But for the resistance [Hezbollah] to enter the army and fight alongside the army, this is not an option, because the army can't fight like the resistance, nor can the resistance fight like the army."
Lebanon's army is drawn from all the country's religious communities and has remained largely neutral in the conflict between Hezbollah and Israel.
However, at least a dozen of its soldiers have been killed after Israeli aircraft targeted its bases.
The increasing signs of a ground war come as Israeli aircraft attacked targets throughout Lebanon for the tenth successive day, bringing the Lebanese death toll in the conflict to more than 340.
Targets across Lebanon were hit on Friday morning, with at least 10 people being killed.
Jets bombed Hezbollah targets in Beirut, the Bekaa valley and southern Lebanon and also hit a previously-bombed bridge on the main Beirut-Damascus highway.
In Baalbek, a city in eastern Lebanon's Bekaa valley, Israeli aircraft bombed Hezbollah buildings, killing at least three civilians, hospital staff said.
In the southern Lebanese city of Tyre, hospital authorities overwhelmed by the number of dead have begun temporarily burying the dead in mass graves.