On Sunday, Israeli warplanes destroyed a makeshift bridge over the Litani river, the last main route for aid agencies to reach the port of Tyre and the rest of the south.
Israel also imposed an indefinite ban on movement south of the river, saying its forces might attack moving vehicles that were not approved convoys, on the grounds that they might be taking supplies to Hezbollah fighters.
The UN World Food Programme (WFP) has asked Israel since Monday for safe passage to rebuild the crossing and look for a new route to the south but has had no luck in either case.
Robin Lodge, WFP spokesman, said on Wednesday that "we haven't yet found a way through. We were unable to send our reconnaissance vehicles down there today because of the security situation.
"We hope to do it tomorrow if we get security assurances."
Lodge said the WFP sought clearance for two planeloads of supplies from Jordan and to bring in about 170 tonnes of aid from Syria, but a similar convoy that arrived on Tuesday had been delayed after Israel destroyed the main highway bridges on the route.
Israel launched an offensive launched against Hezbollah in southern Lebanon after Hezbollah fighters seized two Israeli soldiers in a cross-border raid on July 12.
Relief group Mercy Corps said heavy shelling around the southern town of Nabatiyeh had forced them to call off deliveries from Beirut but it planned to stockpile goods closer so it could move in and out of the area at short notice.
Israeli bombing has destroyed
bridges on main highways
Cassandra Nelson, senior communications officer for Mercy Corps, said that "the security situation is so volatile that one minute it is safe and we are getting green lights to come, and then an hour or so later the place is under attack".
She said aid workers had to switch tactics from sending dry goods south because people had run out of cooking fuel - part of a larger crisis which the health ministry said on Tuesday would force hospitals to close in two or three days.
British aid agency Oxfam said on Wednesday it too was facing huge obstacles in delivering humanitarian aid to Lebanese people hardest hit by the Israeli bombardment.
The international development, relief and campaigning organisation said the humanitarian situation in Lebanon was "appalling" and it was restricted to operating only in the capital Beirut due to fears of their trucks being targeted by Israeli warplanes.
Residents stock up on fuel
containers as a fuel crisis looms
Oxfam said trying to deliver aid to outlying cities was "almost impossible" as the conflict between Israel and the Hezbollah militia in Lebanon entered its fifth week.
Ian Bray, Oxfam's spokesman on humanitarian emergencies,
said hospitals in southern Lebanon were beginning to run out of fuel.
"There are huge challenges to get aid into areas where there is greatest need, which is in the south," he said.
"There are attacks on any movement. We are getting reports that truck drivers are refusing to work because they are afraid of being hit by Israeli missiles - that is causing a huge problem.
"The bombing of roads and infrastructure will hamper any aid effort, even if we sort out the security.
"Hospitals are soon running out of fuel, so getting fuel around that road network is going to be a logistical nightmare."
Oxfam has sent 18 tonnes of water and sanitation equipment to the Middle East.
Villages still occupied
Medical charity Medecins Sans Frontieres (MSF) said that, although Israel had told people to flee southern Lebanon, seemingly deserted villages still held many who were too poor or too frightened to leave.
"When our teams visit villages, which is becoming very, very difficult in itself, even if they look empty there's always civilians left - 10, 50, or maybe 500 in bigger towns," said MSF spokesman Bart Rijs.
He said the agency had been forced to stay put in recent days but was considering moving again soon despite the Israeli imposed curfew.
"We haven't tried to test it yet, but we would like to move out of Tyre again tomorrow," Rijs said.