In a letter to the UN Security Council, Ethiopian Foreign Minister Gedion Timothewos described the incursion as a 'blatant act of aggression that engages Ethiopia's inherent right to self-defence'. The letter followed reports from residents that Eritrean troops had crossed the border days earlier.
Eritrean Information Minister Yemane Gebremeskel responded by accusing Ethiopia of launching an unprovoked attack.
The crisis erupted two weeks ago when fighting resumed between the Ethiopian government and the Tigray People's Liberation Front (TPLF), the party that controls the Tigray region. Both sides accuse each other of violating the 2022 peace deal that ended a two-year civil war.
During that civil war, Eritrea was a key ally of the Ethiopian government, fighting alongside it against the TPLF. Relations have since soured, largely over Ethiopian Prime Minister Abiy Ahmed's ambition to secure access to one of Eritrea's Red Sea ports. Diplomatic ties were severed last week.
Ethiopia's foreign minister said Eritrean forces had advanced up to 60km (37 miles) into Tigray. Ethiopian drone strikes reportedly targeted Eritrean forces in the town of Adigrat on Wednesday and Thursday. The US-based conflict monitor Acled said 'Eritrean troops and ammunition' in Adigrat were hit by 'multiple drone strikes' overnight.
One resident told AFP news agency there were 'many, many drone strikes' and that some Eritrean lorries had been destroyed. Another witness said Eritrean forces appeared to be retreating north after the strikes.