Odaa Tarbii, spokesman for the rebel Oromo Liberation Army (OLA), said on Twitter "The Ethiopian authorities cannot be trusted to hold an impartial investigation."
"There are several atrocities that have warranted investigations in the last four years yet to this day we have heard nothing. What are needed are independent investigations by a third party," he added.
The Ethiopian federal government and the regional authorities in Oromia have blamed the OLA, which is labelled a terrorist group by Addis Ababa, for the massacres in the village of Tole.
Survivors of the Tole attack told AFP that hundreds of people belonging to the Amhara ethnic group were killed by rebels, with bodies still lying on the streets after hours of violence.
Most of the victims were women and children, a UN statement said, adding that at least 2,000 other people had been forced to flee their homes.
No official toll is available.
But the rebel group, which is allied with the Tigray People's Liberation Front (TPLF) in its war against federal forces in northern Ethiopia, has blamed a pro-government militia in the Oromia region.
UN rights chief Michelle Bachelet on Thursday urged the Ethiopian authorities to conduct "prompt, impartial and thorough" investigations into attacks last weekend in the Oromia region that have reportedly claimed the lives of hundreds of people, mostly ethnic Amhara.
On Thursday, the country's parliament instructed the Ethiopian Human Rights Commission, a state-affiliated independent body, to investigate the "inhumane atrocity" it was carried out by the OLA in Oromia, as well as the neighbouring region of Gambella.
The EHRC last week said that security forces had summarily executed residents in the regional capital Gambella, suspecting them of collaborating with OLA rebels who had earlier attacked the southwestern city.