Rebels in south-eastern Ethiopia say they will observe a ceasefire for the week-long visit of a UN delegation.
Ethiopia's Somali region is also known as the Ogaden
The team is probing rights violations and humanitarian issues in the conflict between troops and the Ogaden National Liberation Front in the Somali region.
The ONLF has been fighting for independence for ethnic Somalis in the region for more than 20 years.
The conflict flared in April when the ONLF attacked a Chinese-run exploration team, killing 74 people.
In a statement, the ONLF said its fighters had been instructed to cease all offensive military operations to provide maximum security for the UN team.
Large parts of the region have been inaccessible to outside agencies in recent months as Ethiopian troops attempt to suppress the rebel insurgency.
The BBC's Elizabeth Blunt in Addis Ababa says there are reported to be food shortages, disease outbreaks and serious human rights violations in some areas.
In its statement, the ONLF urged "mission team members to visit all parts in order to investigate war crimes and not limit their mission to the few routes approved by the [governing] regime".
But our correspondent says it is hard to imagine that the UN team will be either willing or able to travel in central Somali region, also known as the Ogaden, without security clearance from the Ethiopian armed forces.
The 14-member UN team includes representatives of the World Food Programme, World Health Organization and the UN Commission for Human Rights.
The ONLF draws its support from Somali clans and accuses the authorities of imposing a blockade on five districts, choking off commercial trade.