By Martin Plaut
BBC Africa editor
International aid agency Medecins Sans Frontiers has accused Ethiopia of denying it access to the country's eastern Ogaden region.
Aid agencies say roads have been closed
The barren region has recently been the scene of a fierce conflict between government troops and rebel forces.
The exclusion follows an order to the Red Cross to stop operations in Ogaden.
The rebels accuse the government of imposing a blockade and creating what they described as a man-made famine. Ethiopia denies imposing no-go zones.
Ogaden, stretching eastwards from the Ethiopian highlands deep into Somalia, is known as Region Five by the Ethiopian government.
A conflict has been raging in the area since April, when fighters of the Ogaden National Liberation Front (ONLF) attacked a Chinese-run exploration team, killing 74 people.
The ONLF, which draws its support from Somali clans, accuses the authorities of imposing a blockade on five districts, choking off commercial trade.
Aid agencies say roads have been closed. Prices are reported to have risen sharply.
An unpublished report by one aid organisation shows that local people produce only a quarter of the food they need, trading their livestock to pay for the rest.
Locals say that the Ethiopians are now escorting some government authorised traders into the area, but there are fears that villagers accused of supporting the rebels may not get access to the food.
Some contraband trade is getting through, on the backs of donkeys, but not in very large quantities.
The UN is now deeply concerned, and has published a map showing the areas of fighting.
It includes areas described as being under a commercial food embargo and one area in which villagers are being forcibly relocated, though the government denies imposing any no-go zones.
A UN team - which is now in the region - should throw more light on what is taking place, if they can have unrestricted access to areas of conflict.