Christopher Stokes, director of operations for the French humanitarian group, said on Monday that the bombing of a makeshift bridge had blocked a convoy of food, medical aid and fuel to Tyre, which has seen an increase in wounded from heavy fighting this week.
"Our last remaining supply route into Tyre into the south has been cut," he said after the air strike on the Qasmiyeh crossing about 10km north of Tyre.
The main Qasmiyeh road bridge was knocked out by earlier air raids.
He said Medecins Sans Frontieres was now considering unloading supplies on one side of the river and shifting them across by hand.
Stokes said this carried extra dangers, especially after an Israeli air strike hit a truck about 40m from a UN convoy, killing two civilians, on Sunday.
"Because the crossing is out, we are going to have to transfer by hand, which will leave us very exposed," he said.
"These are very urgently needed supplies, especially if there is going to be increased fighting near Tyre."
Humanitarian groups, including UN agencies, say Israeli artillery, airstrikes and a naval blockade are hindering them from helping many of the 800,000 to one million people displaced by the war in Lebanon.