As Tigrayan forces celebrated returned control over their regional capital, Mekelle, there was mostly silence from senior Ethiopian officials back in the nation’s capital, Addis Ababa.

As Ethiopian troops left Mekelle and other parts of the Tigray region, the country’s foreign affairs ministry described the retreat as an act of humanitarian good will, saying the government had declared a unilateral cease-fire in order to alleviate the suffering from a looming famine.

“The federal government understands that the people of Tigray, especially those in rural areas, have suffered greatly,” the statement said.

Diplomats briefed on the discussions underway in Addis Ababa said that senior officials within Prime Minister Abiy Ahmed’s government were at odds on how to proceed after claims from senior Tigrayan officials that they would keep fighting Ethiopian troops and their allied Amhara ethnic militias and Eritrean military forces.

Now, two senior Western officials said that Eritrean officials were genuinely concerned that the Tigrayans could reverse the tide of the conflict by invading Eritrea. They said that avoiding another escalation would be the focus for intensive diplomatic efforts.

The effort is expected to include influential African emissaries in the coming days to try to head off more conflict, the officials said.

In Washington, Robert Godec, acting assistant secretary of state for the State Department’s Bureau of African Affairs, said on Tuesday that the Biden administration welcomed the Ethiopian government’s decision to declare a temporary end to hostilities.

“We will watch closely to determine if the cease-fire results in changes on the ground,” he said. “It is essential now that all parties commit to the cease-fire, allow humanitarian access, protect civilians, there be independent investigations into atrocities, and that there is justice for victims.”

“We will not stand by in the face of horrors in Tigray,” he added.

MEKELLE, Ethiopia — In a major turn in Ethiopia’s eight-month civil war in the northern Tigray region, Tigrayan fighters began entering the regional capital Monday night after Ethiopian government troops retreated from the city.

The Ethiopian military has occupied the Tigray region since last November, after invading in cooperation with Eritrean and militia forces to wrest control from the regional government. The Tigrayan forces, known as the Tigray Defense Forces, spent months regrouping and recruiting new fighters, and then in the past week began a rolling counterattack back toward the capital, Mekelle.

New York Times journalists in Mekelle saw thousands of residents take to the streets on Monday night, waving flags and shooting off fireworks after hearing that Tigrayan forces had advanced to the city.

The Tigrayans’ rapid advance was a significant setback for the government of Ethiopia’s prime minister, Abiy Ahmed, who had declared when he sent his forces into the restive Tigray region last year that the operation would be over in a matter of weeks.

A senior interim official who had been installed in Tigray by the federal government confirmed that Tigrayan forces had entered the city and had seized control of the airport and telecommunications network. The official spoke on condition of anonymity to avoid reprisals.

Ethiopia’s government said Monday that it had called a unilateral cease-fire in Tigray. It was not immediately clear, however, whether Tigrayan forces had accepted the truce.

Refugees and international observers have accused the invading forces of wide-ranging atrocities, including ethnic cleansing, and of pushing the region to the brink of famine.

But from the outset, the party in control of Tigray’s regional government, known as the Tigray People’s Liberation Front, or T.P.L.F., which for many years was the ruling party in Ethiopia, has vowed to resist.

Soldiers belonging to the Ethiopian National Defense Forces were seen leaving Mekelle in vehicles throughout the day on Monday, some of them with looted materials, according to international and aid workers. Soldiers also entered the compound of Unicef and the World Food Program, and disconnected the internet, they said. Shops in the city closed early.

On Monday afternoon, the headquarters of the interim government in Tigray were eerily deserted. The only person in the building was a woman working in the cafeteria while her children played on a phone.

Outside the building, federal police officers were seen throwing their belongings onto waiting buses and hastily getting ready to leave.

At the Axum Hotel, where some of the interim leaders had been staying, a receptionist said that the officials had checked out on Sunday and left. By Monday, some of them were already back in the Ethiopian capital, Addis Ababa.

The Ethiopian government has kept up a harsh communications blackout from Tigray, and for months it was unclear what was going on in the region aside from scattered reports of continued fighting, and a growing stream of reports of rape, extrajudicial killings and other atrocities as victims came into Mekelle for treatment.

That shifted over the past week, as escalating violence and troop movements in Tigray made clear that the Tigrayan forces were on the counterattack. Heavy weapons were part of the fighting on both sides, and key towns reportedly changed hands among Ethiopian, Eritrean and Tigrayan forces, U.N. security documents show.

Just a day later, Tigrayan rebels struck back, downing an Ethiopian Air Force C-130 cargo plane as it approached Mekelle. In the days since, reports of rebel victories and Ethiopian government retreats began increasing.

Ethiopian forces reportedly abandoned a number of strategic positions around Adigrat, Abiy Adiy and in several locations in southern Tigray. The rebels also say they have captured several thousand Ethiopian soldiers and are holding them as prisoners of war.

Though the Tigrayans appear to have the upper hand, for now, around Mekelle, the picture in the rest of the region is still murky.

Despite Eritrea’s announcement in March that it would withdraw its forces from Tigray, Eritrean troops have continued to be a factor in the fighting. Eritrean forces have been spotted by aid workers throughout the Tigray Region, from towns in the far north west to dwellings in central areas of the region, where they have bolstered federal forces loyal to Mr. Abiy for months.

It was also unclear whether diplomatic efforts by other countries were having an effect on the action in Tigray. On Saturday, Ethiopia’s deputy prime minister, Demeke Mekonnen, met with several senior diplomats from Britain, Germany, the United States and Spain in Addis Ababa, according to two diplomats briefed on the talks. The diplomats said those talks did not reach any consensus on the need for a cease-fire in Tigray.

“We have launched an offensive at the divisions which we believed were critical,” Mr. Getachew said. “At the same time, they have abandoned many towns and cities.”

Declan Walsh reported from Mekelle, Ethiopia, and Simon Marks from Brussels, Belgium.