ADDIS ABABA, Ethiopia (AP) — Ethiopia’s Nobel Peace Prize-winning prime minister on Wednesday ordered the military to confront the country's Tigray regional government after he accused it of a deadly attack on a military base, declaring “the last red line has been crossed" after months of alleged provocations.

Prime Minister Abiy Ahmed’s move against the well-armed Tigray People’s Liberation Front raised concerns that one of Africa’s most populous and powerful countries could plunge into civil war. That would send a shock wave through one of the world's most turbulent regions, the Horn of Africa. Ethiopia's neighbors include Somalia and Sudan, and the prospect of spreading instability sent a chill down observers' spines.

Signaling the gravity of the threat, the United States in the midst of its election drama issued a statement urging “an immediate de-escalation."

Abiy in a televised address announced “several martyrs” in the overnight attack in Mekele, the northern Tigray region’s capital, and Dansha town. He said “the end is near” for the regional force, based in Ethiopia’s most sensitive region, neighboring Eritrea. The two countries made peace in 2018 after a long border war.

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

The TPLF dominated Ethiopia’s governing coalition before Abiy took office in 2018 and announced sweeping political reforms that won him the Nobel last year. Those reforms, however, opened space for ethnic and other grievances. The TPLF, feeling marginalized by the shifts in power, left the coalition last year. Its strong military force has been reinforced in recent months, but analysts said it's little match for the federal government.

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

A statement on Tigray TV accused the federal government of deploying troops to “cow the people of Tigray into submission by force," and said the Tigray government was acting "to avert more destructive measures.” It banned movement by Ethiopia's military there and warned of “proportional measures” for damage to people or property.

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.” There was no immediate Eritrea comment.

Internet and phone lines were cut in Tigray, provoking distress among people who could not reach loved ones. Tigray TV reported that airspace had been closed over the region, and 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 by President Donald Trump to Africa, and by a multi-layer crisis with the COVID-19 pandemic, deadly ethnic violence and a locust outbreak.

Now the greatest test of Abiy’s rule has come, as the fighting in Tigray could inspire other restive regions in Ethiopia.

Tigray officials had objected to the postponement of Ethiopia’s national election, once set for August, because of the pandemic, and the extension of Abiy’s time in office.

In September, Tigray voted in an election that defied the federal government and increased tensions over a region of some 5 million people that despite its small share of Ethiopia's population of 110 million has had outsize influence. Last month, the federal government moved to divert funding for Tigray to local administrations instead of the regional government, angering the TPLF.

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,” he said. The TPLF calls the release of detained former officials a precondition.

“This war is the worst possible outcome of the tensions that have been brewing,” said William Davison, International Crisis Group’s senior analyst for Ethiopia. “Given Tigray’s relatively strong security position, the conflict may well be protracted and disastrous.”

Abiy’s statement accused the TPLF of arming and organizing irregular militias in recent weeks.

“TPLF has chosen to wage war,” his office said. “The last red line has been crossed with this morning’s attacks and the federal government is therefore forced into a military confrontation” to save the country.

The head of the new state of emergency committee, Redwan Hussein, later told reporters the federal government’s conflict is with a “small clique of TPLF circles that are keen to destabilize Ethiopia," and the government must do everything possible to “liberate the Tigrayan people.” The army was “taking key positions after the initial attack," he said.

Observers have worried for months about the growing tensions and their implications for the Horn of Africa, where Abiy has cast himself as a peacemaker.

A report last month by former U.S. diplomats for the United States Institute of Peace said the fragmentation of Ethiopia “would be the largest state collapse in modern history, likely leading to mass interethnic and interreligious conflict ... and a humanitarian and security crisis at the crossroads of Africa and the Middle East on a scale that would overshadow the existing conflicts in South Sudan, Sudan, Somalia, and Yemen.”

The international community needs to rally around the idea of national dialogue in Ethiopia, the International Crisis Group has said. “The alternative, given the country’s multiple and bitter divides, is a potential march to war that would be catastrophic," it wrote last week.

Fighting could undo Ethiopia's military cohesion, the group said: “Tigray officers hold positions throughout the armed forces in different parts of the country; some might prove more loyal to their region than to federal authorities.”

It was not immediately clear how Ethiopia's regions would respond.

The head of the Amhara region, Temesgen Tiruneh, in a Facebook post said members of Tigray's security forces were welcome to defect, writing that “we know you are fighting because you’ve no choice.” The Amhara and Tigray regions before Abiy's rule were locked in a bitter confrontation over disputed border areas.

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Cara Anna in Nairobi, Kenya, contributed.