Southern Lebanese hospitals were running out of food, fuel and other supplies in today and aid groups said fighting and a ban on movement meant they could not reach an estimated 100,000 people trapped in the area.
Diesel fuel to run generators is also in very short supply.
The UN World Food Programme urged a cessation of hostilities to allow aid supplies to reach the needy.
"Above all, we require a cessation of hostilities by both sides to allow humanitarian aid through," Zlatan Milisic, WFP emergency coordinator in Lebanon, said in a statement. "Our aid operation is like a patient starved of oxygen, facing paralysis, verging on death, if we can't open up our vital supply lines."
He said the destruction of bridges and Israel's refusal to give safety guarantees to aid convoys was crippling the efforts of WFP, which is coordinating the UN relief effort in Lebanon, to organise overland transport of aid.
Medecins Sans Frontieres (MSF) said Israel's warning that it might attack any vehicle south of the Litani that had not been given security clearance would worsen the plight of civilians.
"The people in the south are afraid. They are terrified to move," Rowan Gillies, president of MSF International, said in Beirut. "To forbid all forms of movement, without distinction, will lead to even more civilian deaths and suffering."
MSF said it had suffered close calls with shelling and air strikes close to two of its convoys earlier this week. On Monday, warplanes attacked two cars travelling near a UN convoy, killing three people.
Israel has drawn international criticism for attacking targets in populated civilian areas. At least 1,011 people have been killed in Lebanon during the four-week-old conflict with Hizbollah guerrillas.
Israel, which has lost 121, mostly soldiers, says air attacks and ground operations are the only way to stop the Shi'ite group, which sparked the latest conflict when it captured two Israeli soldiers in a cross-border raid on July 12th.
WFP said it had sent a 15-truck convoy to the eastern town of Baalbek and was trying to send a 10-truck convoy to Nabatiyeh in the south, but had not received security guarantees. The convoy would travel on Friday if it got the go-ahead.
It said two ships carrying aid were due in Beirut at the weekend, one carrying high energy biscuits and the other carrying 2,750 tonnes of wheatflour, pasta and pulses.
The shortages coincide with heavy fighting that has brought new wave of casualties to southern hospitals. More than 3,000 people have been wounded in Lebanon so far and the United Nations says up to 900,000 people have been displaced.
"We're trying to reduce the number of people who have been wounded turning into people who have died," said Gillies.
"It's very basic. If we can't give the local authorities the ability to do that, the consequences for civilians are dire."