Two senior Ethiopian army officers have defected to join the Oromo Liberation Front (OLF), the rebel group says.
Brig Gen Kemal Geltu defected last month
The two are Brig Gen Hailu Gonfa and Col Gemechu Ayana, who commanded the Eighth Mechanized Division.
However, Ethiopian officials said they had no knowledge of the reports. Last month Brig Gen Kemal Geltu also joined the Eritrea-backed OLF.
The two men said it was time for them to take sides in the fight between tyranny and liberty, the OLF says.
Government critics say the defections are a sign of its unpopularity but the government has portrayed the defectors as malcontents.
Gen Kemal said he had crossed the border to Eritrea, along with "hundreds" of men.
The OLF, which was at first a secessionist movement, says it is fighting for more rights for the Oromo people in Ethiopia.
The Oromo are the largest ethnic group in Ethiopia, comprising about half of its 70m people.
Demonstrations after Prime Minister Meles Zenawi was re-elected in May 2005 led to a crackdown on many Oromo and other opposition organisations.
Ethiopia and Eritrea ended a border war in 2002, but Ethiopia has not withdrawn troops from the town of Badme which arbitrators awarded to Eritrea, and tensions between the two countries remain high.