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.