The UN has appealed to Kenya to allow food aid for more than 100,000 people through its border to war-torn Somalia.
The stranded food would feed 100,000 people for three months
The 140 trucks have been stranded on the border for nearly a month, the UN's World Food Programme (WFP) says.
Kenya closed its border with Somalia in January to people and commercial traffic but humanitarian assistance has previously been allowed to across.
Thousands have fled continued unrest around the capital, Mogadishu, where a curfew comes into force on Friday.
Ethiopian and government troops ousted the Union of Islamic Courts (UIC), the Islamist group that controlled most of Somalia for six months last year, in December.
The UN has warned of rising malnutrition rates in Somalia where it plans to assist more than 1m people.
The WFP says their contracted trucks left the Kenyan port of Mombasa and were unexpectedly stopped when trying to cross at El-Wak.
"The Kenyan overland route was chosen because of major problems with sea routes to Somalia plagued by pirate attacks," said the WFP's Peter Goossens.
"Delays in distributing food this month to 108,000 people in Gedo district risks further aggravating the alarming rates of malnutrition that are already reported there."
Many trucks have waited so long at the border that they have been unloaded in recent days and the food moved to a local warehouse, the UN says.
In Gedo region, which borders Kenya, acute malnutrition rates of 15-20% were reported in April.
Earlier this month, the UN's Food and Agriculture Organisation warned that malnutrition was increasing in the Middle and Lower Shabelle regions - areas around the capital.
Meanwhile, in Mogadishu, a huge roadside bomb exploded near the main seaport killing four policemen and one civilian on Friday morning.
The curfew, which will run between 1900 (1600 GMT) and 0500 (0200 GMT), comes into effect on Friday and will continue indefinitely.
Ethiopian troops are conducting a weapons' search in city's main Bakara market where eight people were killed on Thursday.
Correspondents say more than 10 people have died in fierce fighting in Kismayo between clan militia seeking control of the southern port.
Explosions have also been reported in the central Bay and Hiraan regions.
BBC Africa analyst Mary Harper says the overall picture for southern and central Somalia is of a country collapsing into conflict.
The area previously controlled by the UIC is fragmenting into a patchwork of unstable regions, each beset with insecurity, she says.
The violence does not appear to be co-ordinated and it is rare for anybody to claim responsibility for the attacks.
It is unlikely that a national reconciliation conference due to start in Mogadishu next month will have an immediate effect on the situation.
Islamist leaders and a growing number of other Somali groups say they will not take part in any peace negotiations until the Ethiopians leave their country.