Rations have already been cut by a third since July last year
The UN has warned that it has run out of food to provide for nine million Ethiopians who rely on its assistance.
A UN spokesman told the BBC the port of Djibouti was seriously congested and there was little prospect of supplies arriving for the next five months.
Following a border war, Ethiopia does not use Eritrean ports and the landlocked country relies on Djibouti.
However, Ethiopia's ambassador to the UK has denied that his country is facing a food crisis.
Berhanu Kebede told the BBC's World Today programme that food was being transported by truck from Sudan.
Correspondents say this time of year is known as "the hunger season", three months before the next harvest.
The UN World Food Programme says breast-feeding mothers, children and refugees will be among those worst hit.
It warns after it hands out final rations this month there will be no further deliveries until September or October.
The agency says it has no option but to cut back on the food they provide, which has already been cut by a third since July 2008.
"We have a small refugee population here and their ration is being cut by half beginning this month. We run out of food and people will be very hungry," WFP's Barry Came told the BBC.
BBC Africa analyst Martin Plaut says in the jargon of the aid agencies, the food pipeline has ruptured.
The port of Djibouti is full to overflowing and the Ethiopian government has prioritised the delivery of fertiliser, to try to increase the next harvest.
But even when the grain gets through, the WFP says there is an acute shortage of trucks, with the Ethiopian authorities preventing the agency from bringing in its own fleet from Sudan.