Ethiopia's government said it had no option but to defend itself against what it called an "all-out invasion" by Eritrean forces, in a letter to the UN Security Council shared with Reuters on Friday. 

In the government's first official statement on reported incursions by Eritrean troops this week that have raised fears of outright war between the Horn of Africa neighbours, Ethiopia's Foreign Minister Gedion Timothewos said that the Eritrean forces had deployed up to 60 km into northern Ethiopia.

The letter was shared with Reuters by a senior Ethiopian government official. 

Eritrea’s information minister Yemane Gebremeskel did not immediately respond to a request for comment on Friday. On Wednesday ⁠he called reports of an Eritrean incursion part of an Ethiopian plan to justify a war of aggression against his country. 

Ethiopia's letter called the presence of Eritrean troops on its soil "a blatant act of aggression that engages Ethiopia's inherent right to self-defence." 

"Taking its belligerence and intransigence to a whole new level, the Eritrean government has deployed its troops, including mechanised units, deep into Ethiopian territory," the letter said. "The all-out invasion of parts of northern Ethiopia by Eritrea is a matter of public record." 

Eritrean soldiers were seen in northern Ethiopia this week, witnesses told Reuters, days after the neighbours severed diplomatic ties.

Ethiopia has accused Eritrea of backing a rebel alliance battling federal government troops since September 23 in the northern regions of Tigray, Amhara and Afar. Eritrea has denied this.