Abiy’s government is mobilising troops from around the country and sending them to Tigray after two days of clashes between federal forces and the Tigray People’s Liberation Front (TPLF), long the most powerful political force in the country.

The operations sought “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,” Abiy said.

A key objective was to “disarm any security force of the regional state,” the prime minister’s office added in a statement that accused the TPLF of hiding in the regional capital Mekelle and “using the civilian population as human shields”.

“Members of the TPLF, who ruled the country for the previous 27 years through means of oppression rather than law, have been fugitives from justice,” it said, adding that those suspected of illegal activities would be detained.

TPLF officials were not immediately available for comment.

The TPLF took power in Ethiopia in a revolution in 1991 and ruled as the most powerful faction in a multi-ethnic coalition until 2018, when Abiy took office. The prime minister has since reorganised the coalition into a single ruling party, which the TPLF refused to join.

Diplomats have tried to push the sides towards negotiations this week to prevent a civil war.

“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,” U.N. Secretary-General António Guterres said in a message on Twitter late on Thursday.

Abiy, who has tried to open up what has long been one of the most restrictive political systems in Africa, won the Nobel Prize for ending a conflict with neighbouring Eritrea. But he has failed to tamp down ethnic violence in a country divided into regions run by powerful local chiefs.

Sporadic sounds of shelling could be heard from Abdurafi town, near the border between Tigray and the neighbouring Amhara region, at 3 a.m local time (0000 GMT) on Friday, a humanitarian worker in the area told Reuters.

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.

The Tigray regional administration, which is led by TPLF chairman Debretsion Gebremichael, has said it is well equipped to deal with an attack from any direction.

It said this week that it had taken over assets of the national defence force’s northern command, one of the most heavily armed parts of the force, based in Tigray. Two diplomats and a regional military officer said the claim was credible.

“(The) conflict could test the national military’s cohesion, putting particular stress on the Northern Command, which is based in Tigray,” the International Crisis Group think tank said in a briefing.

Federal troops are being helped by local forces from Amhara, its regional president, Temesgen Tiruneh, said in a Facebook post on Thursday. The two regions have been locked in a boundary dispute.