An air strike destroyed the last crossing over the Litani river in Lebanon today, cutting off the main artery for aid to the southern port of Tyre and hard-hit areas nearby.
Christopher Stokes, director of operations for Medecins San Frontieres (MSF), said the bombing of the makeshift bridge at Qasmiyeh had blocked a convoy of food, medical aid and fuel to Tyre, which has seen an increase in wounded from heavy fighting in the past week.
"[The Israelis] have also told us that they can provide no security guarantee that our convoy will not be attacked, so if we move it will be at our own risk and peril," Mr Stokes said.
Aid efforts would continue, he said, as a United Nations convoy of 19 trucks carryied aid south of Beirut to Sidon and 11 trucks brought supplies south from Syria on a slow, difficult route after Israeli jets destroyed four bridges on what the UN called its aid "lifeline" on Thursday.
The UN has led the wide array of complaints by humanitarian groups who say Israeli artillery, air strikes and a naval blockade are hindering them from helping many of the 800,000 - 1,000,000 people displaced by the war in Lebanon.
Patrick Renauld, head of the European Commission's delegation in Beirut, said the EU was deeply concerned by air strikes against civilian infrastructure and expressed shock at the lengths humanitarian groups had to take to protect themselves.
"What kind of world are we living in where we have to use armoured vehicles to deliver aid.
"The fact that all the bridges and roads are broken is creating huge difficulties ... We cannot work in a humanitarian capacity unless international rules are followed."
Israeli air strikes have destroyed around 70 bridges, as well as roads, airports and ports in damage estimated at more than $2.5 billion.