Israeli warplanes wrecked four bridges along the coastal highway in the Christian heartland north of Beirut, cutting off practically the last landlink with neighbouring Syria.
"This was the main supply route for UNHCR, all our supplies come from Syria," said Jennifer Pagonis, a spokeswoman for the office of the UN High Commissioner for Refugees.
The UNHCR, backed by other UN relief agencies and the International Organisation for Migration (IOM), said the humanitarian crisis had deepened after the bombings.
"The road was badly damaged.. with the result that we cannot bring assistance from Arida, from Syria to Beirut," said Christiane Berthiaume, a spokeswoman for the UN's World Food Programme (WFP).
A UN team was unable early Friday to evaluate the damage caused by the air raids or to find new secondary routes from Syria to Lebanon because Israeli military forces did not give security clearance, known as "concurrence", Berthiaume added.
"We did not have concurrence to be able to check