Ethiopia’s prime minister has acknowledged for the first time that troops from Eritrea have been in the Tigray region following the outbreak of conflict in November.
For months both countries have denied that troops crossed the border.
Abiy Ahmed told MPs that Eritrean forces came fearing they would be attacked by Tigray’s regional fighters.
The conflict began after the Tigray People’s Liberation Front (TPLF) captured military bases in the region.
The TPLF had been the ruling party in the northern Ethiopian region, but had a massive fall-out with Mr Abiy over the future of Ethiopia’s ethnically based federal system and its role in government.
Hundreds of thousands of people have fled their homes in Tigray over the last five months.
Despite the TPLF being ousted from power in Tigray at the end of November and Mr Abiy declaring that the conflict was over, fighting is continuing in parts of the region.
‘Any damage is unacceptable’
Mr Abiy, a former Nobel Peace Prize winner, did not admit Eritrean soldiers had been deployed in Ethiopian towns.
Rights groups allege Eritrean soldiers have committed atrocities in towns like Aksum, which include killing unarmed civilians, raping women and the widespread looting of public and private properties.
The prime minister told parliament that Eritrea had said its soldiers were acting to secure the border, taking over trenches there abandoned by Ethiopian soldiers who had gone off to fight.
He said he had spoken to Eritrean officials about allegations that soldiers from Eritrea had carried out atrocities in Tigray.
“After the Eritrean army crossed the border and was operating in Ethiopia, any damage it did to our people was unacceptable,” the AFP news agency quotes him as saying.
His Twitter account added: “Reports indicate that atrocities have been committed in Tigray region.
“Regardless of the TPLF propaganda of exaggeration, any soldier responsible for raping our women and looting communities in the region will be held accountable as their mission is to protect.”
To many observers it has long been clear that Mr Abiy struck a deal with the Eritrean president to combine forces and oust a common enemy, BBC World Service Africa editor Will Ross reports.
Last week, the UN said it would work with the state-linked Ethiopian Human Rights Commission to investigate reports of the indiscriminate killing of civilians and widespread sexual abuse.
The US has described the violence in Tigray as ethnic cleansing.
Eritrea has dismissed accusations of abuses – in particular those detailed in Aksum – as “preposterous” and “fabricated”.
At the end of November, Mr Abiy had told parliament that “not a single civilian was killed” during the conflict.
A communications and electricity blackout and restricted access to Tigray has meant reports of what has gone on have been slow to emerge.