Soldiers from the Ethiopian army have been accused of taking sides during three days of ethnic fighting in the western Gambella region last December.
The region's former President, Okello Akuaye, told the BBC's Network Africa programme that civilian highlanders, supported by the military, attacked local people.
"A few soldiers were leading the groups doing the killing," he said.
Mr Okello Akuaye - who fled the area in the aftermath of the clashes and is now seeking political asylum in Norway - said eyewitnesses had confirmed his allegations.
The Ethiopian federal government has denied any military involvement in the attacks, saying that troops were only there to keep the peace.
The government described the fighting as between local ethnic Anuaks and highlanders.
The UN refugee agency, UNHCR, at the time estimated that 30 people died in the clashes - including three refugee workers and two policemen, although other estimates put the figure at 300.
A month later the government announced that another 200 people had died in further clashes, involving the Anyuak and traditional gold miners.
Some 10,000 people fled the fighting.
Tension in the Gambella region has been simmering for years, but the region is isolated and communications are poor.