Ethiopia's government has been accused of forcing thousands of civilians from their homes in a campaign against separatist rebels in the south-east.
US-based Human Rights Watch says troops had burned homes, seized livestock and carried out arbitrary arrests.
The group also accused the rebels of targeting officials and clan leaders who failed to support its insurgency.
Security was stepped up in April in the remote Somalia region after a deadly rebel raid on a Chinese-run oil field.
The ethnic Somali rebels - the Ogaden National Liberation Front (ONLF) - have been fighting for secession for the Somali region, also known as the Ogaden, since the 1990s.
"The Ethiopian government appears to be pursuing an illegal strategy of collective punishment of the civilian population, and the ONLF has targeted civilians for attack," Human Rights Watch's Africa director Peter Takirambudde said.
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
"Whatever the military strategy behind them, these abuses violate the laws of war," his statement continued.
But Bereket Simon, special adviser to Ethiopia's prime minister, denied the accusations.
"The facts on the ground are absolutely different from what they say," AP news agency quotes Mr Bereket as saying.
HRW's Tom Porteous told the BBC's Focus on Africa programme that the report was based on evidence collected from people fleeing the region, mainly to neighbouring Somaliland.
"Whatever the sympathies of those civilians in the Somali region, it is quite clear under international humanitarian law that targeting these civilians is illegal and unacceptable," he said.
"In the long run it's counter-productive; it's likely to lead to an escalation of the conflict rather than a de-escalation of it. This conflict has serious regional implications."
Meanwhile, about 100 Somalis are reported to be stranded on the border after being deported from Ethiopia, but refused permission to enter Somaliland.