Aid agencies in Lebanon say the severe damage to roads and bridges by Israeli air strikes is having a drastic effect on distribution of badly needed aid.
Goods can only be transported in small lorries for fears that larger vehicles will be targeted, they say.
Israel's army has reportedly been told to make plans for a major push into Lebanon, as far as the Litani river.
Lebanon says more than 900 people - mostly civilians - have died. Israel has lost 63 people - 24 civilians.
Eight Israeli civilians and four soldiers were killed in Hezbollah attacks on Thursday, and three members of a Lebanese family died in an Israeli attack on a village.
The Israeli campaign began three weeks ago after Hezbollah militants captured two Israeli soldiers.
A United Nations resolution calling for a truce appears near to completion.
Latest Mid-East crisis map
The UK, France and the US hope to present the first part of a two-stage peace plan to the other 12 members of the UN Security Council later on Thursday.
A second resolution including authorisation for an international peacekeeping force in southern Lebanon may be proposed later.
Since such a force could take weeks or months to arrive, a smaller force of French soldiers may be sent in first, BBC world affairs correspondent Frank Gardner notes.
Israeli Prime Minister Ehud Olmert has said there will be no ceasefire until an international force is deployed in southern Lebanon.
In other developments:
Aid workers are warning of worsening conditions among refugees in the southern border region.
The UN's refugee agency (UNHCR) has warned that fuel shortages continue increasingly to hamper humanitarian relief operations.
Aid agencies told a news conference in Beirut that damage to the main roads meant that aid convoys had to take the more perilous mountain route to the south.
"Roads into these towns are demolished, they're bombed, bridges are destroyed," Jamsheed Din from the British charity Islamic Relief told the BBC.
"They've got no way of getting out. It's increasingly difficult to get in. They're trapped, and the situation is just terrible."
The agencies say they are also using small vehicles for fear that larger ones will be hit by Israeli aircraft searching for Hezbollah fighters.
'Million displaced'
Israeli officials said Defense Minister Amir Peretz had told top army officers to begin preparing for a push to the Litani, which is up to 30km north of the Israeli border.
Any such operation will require approval by Israel's seven-member security cabinet.
The Israeli air force said it had carried out 70 raids on Lebanon overnight, hitting the southern Beirut suburb of Dahieh, a Hezbollah stronghold.
There were also air strikes in the southern town of Nabatiyeh, on a bridge in the northern region of Akkar and roads near the border with Syria, and in the Bekaa Valley.
Israeli planes have been dropping leaflets on southern Beirut telling residents to leave.
One of the leaflets dropped on Beirut on Thursday evening read: "The Israeli army intends to expand its operations in Beirut".
Leaflets were dropped on the suburbs of Haret Hreik, Bir Abed, Hay Madi and Roweiss.
In southern Lebanon the fighting raged on in at least five areas. The three soldiers who died were killed by an anti-tank missile.
More than 100 Hezbollah rockets hit northern Israel on Thursday.
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