Ethiopia’s Nobel Peace Prize-winning prime minister has ordered the military to confront the country’s Tigray regional government after he accused it of carrying out a deadly attack on a military base, declaring “the last red line has been crossed” after months of alleged incitement.

The statement by Abiy Ahmed’s office, and the reported overnight attack by the well-armed Tigray People’s Liberation Front, raised concerns that one of Africa’s most populous and powerful countries could be plunged back into war.

That would send a shockwave through one of the world’s most turbulent regions, the Horn of Africa, where Ethiopia’s neighbours include Somalia and Sudan.

The US, in the midst of its election drama, quickly issued a statement urging “an immediate de-escalation of the current situation in Tigray”.

Addressing the nation on TV, Mr Abiy announced “several martyrs” in the attack in Mekele, Tigray’s capital, and Dansha town. He said “the end is near” for the regional force, based in Ethiopia’s most sensitive region which neighbours Eritrea. The two countries made peace in 2018 after a long border war.

Fighting continued on Wednesday, and the TPLF claimed it had captured and killed Ethiopian army officers, a government statement said later.

The TPLF was the dominant part of Ethiopia’s governing coalition before Mr Abiy took office in 2018 and announced sweeping political reforms that won him the Nobel last year.

Those reforms opened up old ethnic and other grievances. The TPLF, feeling marginalised, left the coalition last year but remains a strong military force, observers say.

Ethiopia declared a six-month state of emergency in Tigray, saying “illegal and violent activities” were “threatening the country’s sovereignty”.

A statement on Tigray TV accused the federal government of deploying troops to “invade Tigray to cow the people of Tigray into submission by force”. It said the regional government had “started to oppose the federal government to avert more destructive measures”.

It banned movement by Ethiopia’s military in Tigray and warned of “proportional measures” for any damage to people or property. It was not clear when the audio statement first aired or who was speaking.

Tigray leader Debretsion Gebremichael on Monday warned a bloody conflict could erupt, accusing Ethiopian and Eritrean leaders of making “all necessary preparations to start war against Tigray”.

Internet and phone lines were cut in the region, and Tigray TV reported that air space had been closed. It asserted that the Ethiopian military’s northern command had defected to the Tigray government. The prime minister’s office told the Associated Press the defection report was “not true”.

Ethiopia was already stressed by a dispute with Egypt over a massive Ethiopian dam project that has drawn rare attention from President Donald Trump to Africa, and by a multi-layer crisis with the Covid-19 pandemic, deadly ethnic violence and a locust outbreak.

Tigray officials objected to the postponement of Ethiopia’s national election in August because of the coronavirus pandemic, and the extension of Mr Abiy’s time in office.

On Sunday, a senior TPLF official, Getachew Reda, told the AP his side will not accept a negotiation with the federal government. “What we need now is a national dialogue, not a negotiation,” he said. The TPLF says the release of detained former officials is one pre-condition.

In September, people in Tigray voted in a local election, defying the federal government and increasing tensions over a region of five million people that, despite its small share of Ethiopia’s population of 110 million, has had outsized influence.

Last month the federal government moved to send funding for Tigray to local administrations instead of the regional government, angering TPLF officials.