NAIROBI, Kenya (AP) — Ethiopia moved Saturday to replace the leadership in the country’s defiant northern Tigray region, where clashes between regional forces and those of the federal government have led to fears that the major African power could slide into civil war. Tigray's leader told the African Union that the federal government was planning a “full-fledged military offensive.”

Neither side appeared ready for the dialogue that experts say is needed to avert disaster.

The upper house of parliament, the House of Federation, voted to set up an interim administration, giving Prime Minister Abiy Ahmed the power to carry out measures against a Tigray leadership his government regards as illegal. Those include appointing officials and facilitating elections.

The Nobel Peace Prize-winning prime minister asserted that “criminal elements cannot escape the rule of law under the guise of seeking reconciliation and a call for dialogue.”

Experts and diplomats are watching in dismay as two heavily armed forces clash in one of the world's most strategic yet vulnerable regions, the Horn of Africa. Observers warn that a civil war in Ethiopia, Africa's second most populous country with 110 million people, could suck in or destabilize neighbors such as Sudan, Eritrea and Somalia.

A statement posted Saturday on the Facebook page of the Tigray government, the Tigray People’s Liberation Front, said it will win the “justified” war, adding that “a fighter will not negotiate with its enemies.”

“Tigray’s people are now armed with modern weaponry that could reach the seat of the infidels,” it said.

In a letter South Africa's president, who is the African Union's chairperson, Tigray leader Debretsion Gebremichael alleged that Ethiopia's federal government and neighboring Eritrea have mobilized their forces near the Tigray border “with the intention of launching a full-fledged military offensive.”

The letter dated Friday, seen by The Associated Press, called Abiy's behavior “unconstitutional, dictatorial and treasonous.”

It said the African Union was well-placed to bring parties to dialogue to “avert an all-out civil war.”

The conflict is playing out between former allies in Ethiopia's ruling coalition who now regard each other as illegal. The TPLF long dominated the country's military and government before Abiy took office in 2018 and introduced sweeping political reforms that won him the Nobel. The changes left the TPLF feeling marginalized, and it broke away last year when Abiy sought to turn the coalition into a single Prosperity Party.

Tensions spiked when the Tigray region, objecting to the delay of the national election until next year and Abiy's extended stay in office, held a local election in September that the federal government called illegal.

Clashes began early Wednesday when Abiy accused the TPLF forces of attacking a military base in Tigray. In a major escalation on Friday, Abiy announced that airstrikes in multiple locations around the Tigray capital “completely destroyed rockets and other heavy weapons” and made a retaliatory attack impossible.

The military operations will continue, the prime minister said, and he warned the Tigray population: ”In order to avoid unexpected peril, I advise that you limit group movements in cities.”

Communications remain almost completely severed with Tigray, making it difficult to verify either side's assertions and leading to pleas from aid groups and human rights groups to warn of a brewing humanitarian disaster. Tigray is increasingly boxed in by Ethiopian federal forces and a six-month state of emergency that curbs public gatherings and movement.

Tigray is preparing for a “major offensive to come tomorrow or the day after from the federal government,” Kjetil Tronvoll, a professor at Bjorknes University College in Norway and a longtime Ethiopia watcher, told the AP. “That's my estimate. Abiy has promised a quick delivery of victory, so he has to move fast.”

Ethiopia's decision on Saturday to replace the Tigray leadership leaves the region with essentially two options, he said: pursuing a “full-out war” with the aim of toppling Abiy's government or declaring independence, especially with Abiy rejecting dialogue.

Encircled, the TPLF can't afford a drawn-out conflict and might fight its way to the capital, Addis Ababa, or toward the Red Sea for an outlet for supplies, Tronvoll said. He described one published estimate of Tigray having nearly a quarter-million various armed forces a “serious underestimate” and said the region's various forces outnumber the federal army “by at least two or three multiples." That's why Addis Ababa has called for reinforcements from other regional forces, he said.

Dino Mahtani with the International Crisis Group, in comments posted Friday, said that if the Tigray forces come under pressure they may even punch into Eritrea, which would then internationalize this conflict." The TPLF and Eritrea have a bitter history of a long border war between Ethiopia and Eritrea before the countries made peace in 2018.

The TPLF's only other real avenue out of Ethiopia is into Sudan, which finds itself “in a very delicate position,” Mahtani said.

Sudan's eastern al-Qadarif province has closed its border with Ethiopia’s Tigray and Amhara regions, the Sudan News Agency reported Saturday.

The Tigray drama dominates conversation in Ethiopia, and many people commenting on social media appear to support the government’s move to get rid of the region's leadership. Some cite abuses by the TPLF while it was in power for well over two decades, or echo the federal government's accusation that it incited recent violence across the country.

Those sympathetic to the TPLF are mostly silenced due to the communications blackout in Tigray, but those able to comment describe the federal government's actions as an aggression that will lead to further escalation of the conflict.

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Elias Meseret in Addis Ababa, Ethiopia, and Samy Magdy in Cairo contributed.