The International Committee of the Red Cross (ICRC) has denounced a decision by Ethiopia to expel it from the Ogaden region, allegedly for cooperating with rebels.
The Swiss-run humanitarian agency was on Tuesday given seven days to leave after 12 years operating in the region.
The ICRC said its expulsion would hurt needy civilians.
"All ICRC activities in Ethiopia are conducted in strict accordance with the principles of independence and neutrality," Daniel Duvillard, head of ICRC operations for the Horn of Africa, said on Thursday.
"The ICRC firmly rejects the accusations made against it."
Dialogue between the ICRC and the region's authorities and security forces had deteriorated to a point where "issues of contention were not discussed in depth" before the ejection order was issued, Duvillard said.
The ICRC said it was ready to discuss the matter with Ethiopian authorities so it could continue its humanitarian activities in the Somali regional state, where it deploys ten expatriates and 79 national staff.
Jama Ahmed, vice-president of the Somali region, said the ICRC must "recognise what they did wrong" before officials consider allowing it to continue work there.
A spokesman for the regional government said on Wednesday that the ICRC had been repeatedly warned "to desist from a smear campaign against the regional government and from supplying material and finance to a rebel group attempting to destabilise the region".
The ICRC said it carried out its aid work "impartially and on strictly humanitarian grounds".
"A suspension of ICRC activities will inevitably have a negative impact on the population concerned, whose access to basic services will be reduced," it said.
A large proportion of the Somali regional state's population benefited from the ICRC's water and sanitation projects, including efforts to build hand pumps and dig wells, it said.
ICRC officials also train livestock owners and visit detention centres to assess conditions and treatment of inmates.
The expulsion has shocked other humanitarian groups working in the desolate Ogaden area bordering Somalia, where a guerrilla group has accused the Ethiopian authorities of blockading food relief for nearly two months, choking commercial trade and risking "man-made famine".
The authorities announced a crackdown on the rebels in June, two months after the rebels attacked a Chinese-run oil exploration field, killing 74 people.
Rebels said the ICRC's expulsion was designed to prevent the world from "witnessing the war crimes taking place against the civilian population of Ogaden at the hands of the Ethiopian regime".
The UN World Food Program said this week that the military operations was hampering the delivery of humanitarian aid, but that the government was not blockading shipments.
swissinfo with agencies
Ogaden is a part of the Somali Region in Ethiopia. Inhabitants are predominantly ethnic Somali and Muslim.
The region, which is around 200,000 km2 (Switzerland is around 40,000km2), borders Djibouti, Kenya, and Somalia.
In the past, secessionist activities have involved the political goals and militaries of Ethiopia and Somalia. In the late 1970s both countries fought the Ogaden War over control of the region and its peoples.
The Somalis who inhabit Ogaden claim that the Ethiopian military kill civilians and destroy the livelihood of many of the ethnic Somalis.
Currently the main separatist group is the Ogaden National Liberation Front, which is fighting to liberate the Ogaden from what they see as Ethiopian occupation.