Ethnic-Somali rebels in the south-east of Ethiopia say they have launched an offensive against government forces and captured several towns.
The Ogaden National Liberation Front (ONLF) said it began attacking on several fronts on Tuesday.
The separatists said a "significant number" of Ethiopian troops had been killed and their equipment captured.
The reports could not be verified and Ethiopia has in the past dismissed rebel accounts of military gains.
"The operation involved thousands of ONLF troops and resulted in two days of heavy fighting," an ONLF statement said.
The group added that its forces had been "warmly welcomed" in the towns it claimed to have captured - Obolka, Hamaro, Higlaaley, Yucub, Galadiid, Boodhaano and Gunogabo.
The ONLF, formed in 1984, is fighting for the independence of ethnic Somalis in the oil-rich Ogaden region.
It says the Somali-speaking population has been marginalised by Addis Ababa.
Fighting has escalated over the past two years following an ONLF attack on a Chinese-run oil exploration field.
More than 70 people died in the attack, including Ethiopian guards and Chinese workers.
Addis Ababa calls the rebels "terrorists" and has cut off all access to the region.
However, watchdogs have accused the Ethiopian government of human rights violations.