The two main rebel groups in eastern Ethiopia say their forces have killed dozens of Ethiopian soldiers in joint operations in May in the Ogaden region.
But senior Ethiopian official Bereket Simon said their claims were untrue and just an attempt to get media attention.
A statement from the Oromo Liberation Front said several separate attacks had led to more than 150 soldiers killed.
The BBC's Amber Henshaw in Addis Ababa says there is high tension in the area since an attack on a Chinese oilfield.
But she says the remoteness of the location means it is unclear what is going on there.
OGADEN NATIONAL LIBERATION FRONT (ONLF)
Want Somali-speaking region to break away from Ethiopia
Founded in 1984
Has been accused of bomb attacks in Somali region and the capital, Addis Ababa
Fought major battles with Ethiopian government in 2006
The nationalist OLF springs from Ethiopia's largest ethnic group in the east while the other group, the Ogaden National Liberation Front (ONLF), is made up mostly of people of Somali origin fighting for a separate state from Ethiopia.
Last month, the ONLF attacked a Chinese-run oil installation in the region, killing 65 Ethiopian and nine Chinese workers.
The ONLF representative in the UK Abdurahman Mahdi said they had launched a joint operation with the OLF as a defensive manoeuvre.
Mr Bereket, Ethiopia's former information minister and now a special adviser to the prime minister, told the BBC the claims were "completely untrue" but said there was a "serious hunting operation" under way following the oil-field attack.