Ethiopia has accused Eritrea of an "all-out invasion" of its northern Tigray region and warned it will defend itself, days after residents told the BBC Eritrean troops had crossed into the country.
In a letter to the UN Security Council, Ethiopia's foreign minister called it a "blatant act of aggression that engages Ethiopia's inherent right to self-defence", following reported strikes on Eritrean troops in Tigray.
Eritrean Information Minister Yemane Gebremeskel responded by saying it was facing "an unprovoked war of aggression" from its neighbour.
Fighting broke out between the government and the party that controls Tigray, the Tigray People's Liberation Front (TPLF), two weeks ago.
Both blame each other for violating a 2022 deal that ended its civil war. Ethiopia says Eritrea backs the TPLF, which it denies.
During the two-year civil war, Eritrea was a key ally of the Ethiopian government, having fought alongside them against the TPLF.
But relations between the two neighbours have since deteriorated - largely over the Ethiopian prime minister's ambition to take ownership of one of Eritrea's Red Sea ports - leading to the severing of diplomatic ties last week.
Accusing Eritrea of violating its sovereignty, Ethiopian Foreign Minister Gedion Timothewos said Eritrean forces had advanced up to 60km (37 miles) into Tigray.
Ethiopian drone strikes reportedly targeted Eritrean forces in the Tigrayan town of Adigrat on Wednesday and Thursday, amid fears of a wider regional war.
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, AFP reported.
There have so far been no verified reports of casualties. The BBC is unable to report from Tigray.
Dated 8 October, Ethiopia's letter to UN Security Council members read: "In this time of difficulty, when Ethiopia is left with no other option but to defend itself, we count on your understanding, support, and solidarity."
But while Ethiopia accuses its neighbour of supporting the TPLF, Eritrea has blamed the conflict on Ethiopia's leaders, and accused them looking for a reason to seize Eritrean territory.
On Friday, Eritrea's information minister called the accusations levelled against Asmara as "duplicity in their most extreme forms".
In a post on X, he said Ethiopia was "openly working to bring about regime change in Eritrea by providing shelter and support to certain mercenaries who claim to be opposition".
Eritrea gained independence from Ethiopia in 1993, but ties were only normalised in 2018, when Ethiopian Prime Minister Abiy Ahmed came to power.
This latest unrest follows months of tension and the launch of new alliance of armed groups in Ethiopia - including the TPLF - in September.
Last week, TPLF forces withdrew from Tigray's main city of Mekelle, which is 115km (71 miles) south of Adigrat and is now under army control.
Additional reporting by Kaleb Moges