The United Nations has called for immediate de-escalation in Ethiopia's northern Tigray region, where federal troops have been exchanging fire with the powerful ethnic faction that led the ruling coalition for decades.
Prime Minister's Abiy Ahmed's government is mobilising troops from around the country and sending them to Tigray, after two days of clashes between government forces and the Tigray People's Liberation Front (TPLF).
"The stability of Ethiopia is important for the entire Horn of Africa region. I call for an immediate de-escalation of tensions and a peaceful resolution to the dispute," UN Secretary General Antonio Guterres said in a message on Twitter.
Mr Abiy said on Friday that the military offensive had limited objectives.
"Operations by federal defence forces under way in northern Ethiopia have clear, limited and achievable objectives – to restore the rule of law and the constitutional order, and to safeguard the rights of Ethiopians to lead a peaceful life wherever they are in the country," he wrote on Twitter.
Sporadic sounds of shelling could be heard from Aburafi town, near the Tigray-Amhara border, at 3am on Friday, a humanitarian worker in the area told Reuters.
Two foreign diplomats briefed on the violence told Bloomberg they have received reports of heavy fighting in three areas that have caused dozens of casualties. The areas include north of the city of Gondor, about 420 kilometres north of the capital, Addis Ababa; the town of Homera; and around a military facility at Dansha.
Mr Abiy has so far resisted diplomatic pressure to halt the military offensive. US Secretary of State Michael Pompeo on Thursday called for “immediate action to restore the peace and de-escalate tensions".
The TPLF was the dominant political force in Ethiopia's multi-ethnic ruling coalition for decades until Mr Abiy, a member of the Oromo ethnic group, took office two years ago.
Mr Abiy, who has tried to open up what has long been one of the most restrictive economic and political systems in Africa, reorganised the ruling coalition into a single party which the TPLF refused to join.
Countries in the region fear that the crisis could escalate into all-out war under Mr Abiy, who won the 2019 Nobel Peace Prize for ending a decades-old conflict with neighbouring Eritrea but has failed to prevent outbreaks of ethnic unrest.
The Tigray administration, which is led by president Debretsion Gebremichael, said on Thursday it was well equipped to deal with an attack from any direction.
Two Ethiopian fighter jets were seen flying over Mekelle, the capital of Tigray, on Thursday afternoon, two diplomatic sources told Reuters, in what was described as a show of force by the Ethiopian National Defence Forces.
Ethiopia closed the airspace over Tigray to all flights on Thursday, and closed all the international and domestic flight routes that traverse its northern airspace.
Sudan closed its border with Ethiopia on concern that clashes may spread across its southern border.
The government of Sudan’s Kassala state said the action was aimed at stopping fighters in Tigray Tigray region from crossing the frontier. Kassala’s governor plans to travel to the border areas to assess the security situation, it said.