After two years of fighting that left hundreds of thousands dead and millions displaced and facing starvation, the surprise deal came out of peace talks convened by the African Union in South Africa.

After two years of brutal civil war, the Ethiopian government and the leadership of the northern Tigray region agreed to stop fighting on Wednesday as part of a deal that offered a path out of a conflict that has killed hundreds of thousands and displaced millions in Africa’s second-most-populous country.

Senior officials from both sides shook hands and smiled after signing an agreement in South Africa to cease hostilities, following 10 days of peace talks convened by the African Union.

The surprise deal came one day before the second anniversary of the start of the war, on Nov. 3, 2020, when simmering tensions between Prime Minister Abiy Ahmed of Ethiopia and the defiant leaders of the country’s Tigray region exploded into violence.

Mr. Abiy, a Nobel Peace Prize laureate, initially billed the war as a “law and order” campaign that he promised would be swift, even bloodless. But it quickly degenerated into a grinding conflict accompanied by countless atrocities, including civilian massacres, gang rape and the use of starvation as a weapon of war.

The deal was signed by Getachew Reda, a senior leader in the Tigray People’s Liberation Front, and Redwan Hussien, Mr. Abiy’s national security adviser, in Pretoria, South Africa’s administrative capital.

It contained a raft of provisions for disarming fighters, permitting humanitarian supplies to reach Tigray — where five million people urgently need food aid — and bringing a measure of stability to Ethiopia.

“We have agreed to permanently silence the guns and end the two years of conflict in northern Ethiopia,” the two sides said in a joint statement.

But mediators warned that it was just the first step in what would most likely be difficult negotiations before a permanent peace could be achieved. It was unclear how the deal’s provisions would be monitored or carried out. And negotiators cautioned that forces inside and outside Ethiopia could yet derail the process and tip the country back into war.

“This moment is not the end of the peace process,” said Olusegun Obasanjo, the former Nigerian president, representing the African Union, “but the beginning of it.”

Mr. Abiy agreed to the deal at a moment of military supremacy, following weeks of sweeping military advances by his troops across Tigray with the help of allied forces from Eritrea, the neighboring country to the north.

But it also came against a backdrop of loud warnings from the United States and the United Nations about the possibility of new atrocities in a war already scarred by widespread abuses, including ethnic cleansing.

“The situation in Ethiopia is spiraling out of control,” the U.N. secretary general, António Guterres, warned last month.

The scale of fighting in Ethiopia rivals that of Ukraine, the American ambassador to the United Nations, Linda Thomas-Greenfield, said on Oct. 21. As many as half a million people have died as a result of the war in Ethiopia, and the United States was “deeply concerned about the potential for further mass atrocities,” she said.

As African Union-led peace efforts floundered this year, American diplomats brought the two sides together for three secret meetings outside Ethiopia. But while that initiative resulted in a five-month humanitarian cease-fire, it also provided both sides with a chance to rearm for the round of fighting that erupted in August.

After that, American officials, who spoke on condition of anonymity because of the sensitivity of the matter, said they were considering imposing sanctions, authorized by President Biden last year, in an effort to force the belligerents to stop fighting. But they wanted to give the talks a chance.

Ned Price, a State Department spokesman, welcomed Wednesday’s deal as an “important step toward peace.”

Karine Jean-Pierre, the White House spokeswoman, said, “The United States remains committed to supporting this African Union-led process.”

Mr. Abiy hailed it as a “monumental” deal, seeking in a statement to frame it as part of a broader program of “reforms” led by his government. In a less conciliatory note, Ethiopia’s ambassador to Kenya, Bacha Debele Buta, said in a tweet that “Ethiopia has prevailed” and the federal government would rule Tigray “through a command post.”

Still, there was no immediate reaction from Eritrea, which has played a key role in the fighting and which was not formally represented in the peace talks.

The autocratic leader of Eritrea, Isaias Afwerki, has for decades harbored a bitter rivalry with the leaders of Tigray. It was unclear if he had agreed to the deal signed in South Africa and, crucially, if he would withdraw his troops from the region.

Equally uncertain was the political reception by the leaders of Ethiopia’s ethnic Amhara group, who provided crucial political and military support to Mr. Abiy in his campaign against the Tigrayans. They have long claimed that western Tigray, where Ethiopian forces were accused of ethnic cleansing, rightfully belongs to the Amhara region.

The war in Tigray, one of several conflicts in Ethiopia, is rooted in the seismic shifts that have defined the country over the past three decades.

The main Tigrayan group, the Tigrayan People’s Liberation Front, came to power in 1991, when it led a rebellion that ousted a Marxist dictator. It was shunted aside in 2018, after Mr. Abiy, a former ally, came to power amid a popular clamor for change in Ethiopia.

The two sides coexisted uneasily for several years. The Tigrayans retreated to their stronghold in the north of the country. Mr. Abiy built an unlikely alliance with Mr. Isaias, the leader of Eritrea, who had long despised the Tigrayans.

Tensions came to a head in September 2020, when the Tigrayans held regional elections in defiance of Mr. Abiy. Two months later, the feud erupted into war.

The joint agreement signed on Wednesday outlines a plan to allow humanitarian access to Tigray, where a punishing government siege of the region has cut off electricity, banking and other vital services for over 16 months. The deal also has provisions for reintegrating Tigray’s regional government back into the central government, and noted that the Ethiopian government would rebuild all infrastructure damaged in the war.

The two sides also agreed to “a detailed program of disarmament, demobilization and reintegration” of Tigrayan forces, they said.

Analysts said that the coming days would be crucial in determining if the deal could stick, and that much depended on how leaders on both sides sold it to their most vociferous, and heavily armed, supporters.

In South Africa, Uhuru Kenyatta, the former president of Kenya who helped broker the deal, warned in brief comments that unnamed “destructive” actors “from within or without” could scupper the deal — a likely reference to the leader of Eritrea and the Amhara militias that have been fighting alongside the Ethiopian military.

“The devil will be in the implementation,” Mr. Kenyatta said. “We still have a lot of work to do in terms of beginning the political process.”

For civilians across northern Ethiopia, the deal is a welcome respite from a conflict that has become notorious for gross abuses.

At least 2.4 million people have been forced from their homes, according to the United Nations. Rights groups have accused both sides of war crimes including extrajudicial killings, looting and sexual violence. U.N. investigators recently singled out Mr. Abiy’s forces for engaging in “warfare by starvation” and submitting detained women to “sexual slavery.”

American officials said that the same abuses had recurred alongside the recent surge in fighting. After the breakthrough on Wednesday, Mr. Price, the State Department spokesman, said, “It’s our hope that what was announced today will see an end to those reports and, ultimately, the underlying abuses and atrocities.”

For much of the conflict, it has been hard to know exactly what was going on in Tigray. Mr. Abiy’s government cut off the internet and electricity to Tigray in June 2021 and had prohibited reporters from visiting the area. How much that will change now remains unclear.

But the main priority for aid groups, and many international officials, will be to open up access to a region that has undergone immense suffering in the past two years.

The World Health Organization said last week that Tigray had run out of vital medicines, including vaccines and antibiotics. Doctors had resorted to using rags to dress wounds, it said.

In the joint statement released on Wednesday, Mr. Abiy’s government said it would cooperate with humanitarian agencies “to continue expediting aid to those in need of assistance.”

Whether that happens, and how soon, could be an early test of the commitment to peace on both sides.