NAIROBI, Kenya — As fighting raged across the Tigray region of northern Ethiopia last month, a group of soldiers arrived one day at Hitsats, a small hamlet ringed by scrubby hills that was home to a sprawling refugee camp of 25,000 people.

The refugees had come from Eritrea, whose border lies 30 miles away, part of a vast exodus in recent years led by desperate youth fleeing the tyrannical rule of their leader, one of Africa’s longest-ruling autocrats. In Ethiopia, Eritrea’s longtime adversary, they believed they were safe.

But the soldiers who burst into the camp on Nov. 19 were also Eritrean, witnesses said. Mayhem quickly followed — days of plunder, punishment and bloodshed that ended with dozens of refugees being singled out and forced back across the border into Eritrea.

For weeks, Prime Minister Abiy Ahmed of Ethiopia has denied that soldiers from Eritrea — a country that Ethiopia once fought in an exceptionally brutal war — had entered Tigray, where Mr. Abiy has been fighting since early November to oust rebellious local leaders.

In fact, according to interviews with two dozen aid workers, refugees, United Nations officials and diplomats — including a senior American official — Eritrean soldiers are fighting in Tigray, apparently in coordination with Mr. Abiy’s forces, and face credible accusations of atrocities against civilians. Among their targets were refugees who had fled Eritrea and its harsh leader, President Isaias Afwerki.

“Abiy has invited a foreign country to fight against his own people,” said Awol Allo, a former Abiy supporter turned outspoken critic who lectures in law at Keele University in Britain. “The implications are huge.”

Mr. Abiy insists he was forced to move his army quickly in Tigray after the region’s leaders, who had dominated Ethiopia for 27 years until Mr. Abiy took over in 2018, mutinied against his government. But in the early weeks of the fight, Ethiopian forces were aided by artillery fired by Eritrean forces from their side of the border, an American official said.

Since then, Mr. Abiy’s campaign has been led by a hodgepodge of forces, including federal troops, ethnic militias and, evidently, soldiers from Eritrea.

At Hitsats, Eritrean soldiers initially clashed with local Tigrayan militiamen in battles that rolled across the camp. Scores of people were killed, including four Ethiopians employed by the International Rescue Committee and the Danish Refugee Council, aid workers said.

The chaos deepened in the days that followed, when Eritrean soldiers looted aid supplies, stole vehicles and set fire to fields filled with crops and a nearby forested area used by refugees to collect wood, aid workers said. The camp’s main water tank was riddled with gunfire and emptied.

Their accounts are supported by satellite images, obtained and analyzed by The New York Times, that show large patches of newly scorched earth in and around the Hitsats camp after the Eritrean forces swept through.

Later, soldiers singled out several refugees — camp leaders, by some accounts — bundled them into vehicles and sent them back across the border to Eritrea.

“She’s crying, crying,” said Berhan Okbasenbet, an Eritrean now in Sweden whose sister was driven from Hitsats to Keren, the second-largest city in Eritrea, alongside a son who was shot in the fighting. “It’s not safe for them in Eritrea. It’s not a free country.”

Mr. Abiy’s spokeswoman did not respond to questions for this article. However, a few weeks ago the United Nations secretary general, António Guterres, bluntly asked Mr. Abiy if Eritrean troops were fighting in his war. “He guaranteed to me that they have not entered Tigrayan territory,” Mr. Guterres told reporters on Dec. 9.

Those denials have been met with incredulity from Western and United Nations officials.

The Trump administration has demanded that all Eritrean troops immediately leave Tigray, a United States official said, citing reports of widespread looting, killings and other potential war crimes.

It remains unclear how many Eritreans are in Tigray, or precisely where, said the official, who spoke on the condition of anonymity to discuss delicate diplomacy. A communications blackout over Tigray since Nov. 4 has effectively shielded the war from outside view.

But that veil has slowly lifted in recent weeks, as witnesses fleeing Tigray or reaching telephones have begun to give accounts of the fighting, the toll on civilians and pervasive presence of Eritrean soldiers.

In interviews, some described fighters with Eritrean accents and wearing Ethiopian uniforms. Others said they witnessed televisions and refrigerators being looted from homes and businesses. A European official, speaking on the condition of anonymity to discuss confidential findings, said some of those stolen goods were being openly sold in the Eritrean capital, Asmara.

The reports of Eritrean soldiers sweeping through Tigray are especially jarring to many Ethiopians.

Ethiopia and Eritrea were once the best of enemies, fighting a devastating border war in the late 1990s that cost 100,000 lives. Although the two countries are now officially at peace, many Ethiopians are shocked that the old enemy is roaming freely inside their borders.

“How did we let a state that is hostile to our country come in, cross the border and brutalize our own people?” said Tsedale Lemma, editor in chief of the Addis Standard newspaper. “This is an epic humiliation for Ethiopia’s pride as a sovereign state.”

Mr. Abiy has already declared victory in Tigray and claimed, implausibly, that no civilians have died. But last week his government offered a $260,000 reward for help in capturing fugitive leaders from the regional governing party, the Tigray Peoples Liberation Front — a tacit admission that Mr. Abiy has failed to achieve a major stated goal of his campaign.

In fact, the biggest winner so far may be his Eritrean ally, Mr. Isaias.

Since coming to power in 1993, Mr. Isaias has won a reputation as a ruthless and dictatorial figure who rules with steely determination at home and who meddles abroad to exert his influence.

For a time he supported the Islamist extremists of the Shabab in Somalia, drawing U.N. sanctions on Eritrea, before switching his loyalties to the oil-rich — and Islamist-hating — United Arab Emirates.

The peace pact signed by the two leaders initially raised hopes for a new era of stability in the region. Ultimately, it amounted to little. By this summer, borders that opened briefly had closed again.

But Mr. Abiy and Mr. Isaias remained close, bonded by their shared hostility toward the rulers of Tigray.

They had different reasons to distrust the Tigrayans. For Mr. Abiy the Tigray People’s Liberation Front was a dangerous political rival — a party that had once led Ethiopia and, once he became prime minister, began to flout his authority openly.

For Mr. Isaias, though, it was a deeply personal feud — a story of grievances, bad blood and ideological disputes that stretched back to the 1970s, when Eritrea was fighting for independence from Ethiopia, and Mr. Isaias joined with the Tigray People’s Liberation Front to fight an Ethiopian Marxist dictator.

Those differences widened after 1991, when Eritrea became independent and the Tigrayans had come to power in Ethiopia, culminating in a devastating border war.

“It’s typical Isaias,” said Mr. Plaut. “He seeks to project power in ways that are completely unimaginable for the leader of such a small country.”

Aid groups warn that, without immediate access, Tigray will soon face a humanitarian disaster. The war erupted just as villagers were preparing to harvest their crops, in a region already grappling with swarms of locusts and recurring drought.

Refugees are especially vulnerable. According to the United Nations, 96,000 Eritrean refugees were in Tigray at the start of the fight, although some camps have since emptied. An internal U.N. report from Dec. 12, seen by The Times, described the situation at Hitsats as “extremely dire,” with no food or water.

Farther north at Shimelba camp, Eritrean soldiers beat refugees, tied their hands and left them under the sun all day, said Efrem, a resident who later fled to Addis Ababa, the Ethiopian capital.

“They poured milk on their bodies so they would be swarmed with flies,” he said.

Later, Efrem said, the soldiers rounded up 40 refugees and forced them to travel back across the border, to Eritrea.

Declan Walsh reported from Nairobi, Kenya, and Simon Marks from Addis Ababa, Ethiopia. contributed reporting from Washington, an Christiaan Triebert from New York