The Ethiopian army pulled out of the Tigray regional capital of Mekelle because it was no longer a "centre of gravity for conflicts", Prime Minister Abiy Ahmed said, as a government official added that troops could return within weeks if needed.

It was the first public statement by a federal government official since Mekelle was taken by Tigray People's Liberation Front (TPLF) forces this week in a major turn of events after eight months of conflict in which thousands have been killed.

People in Mekelle, where communications were down on Wednesday, said two days earlier that incoming Tigrayan fighters had been greeted with cheers. There were similar scenes in the northern town of Shire on Wednesday, where Eritrean troops had pulled out and Tigrayan forces had entered, residents said.

People celebrated in the streets of Shire as they welcomed the Tigrayan forces, a resident who witnessed their arrival told Reuters on condition of anonymity.

Abiy confirmed the withdrawal from Mekelle.

"When we entered Mekelle seven or eight months ago, it was because it was the centre of gravity for conflicts," he told state media on Tuesday in a video posted on his website on Wednesday.

"It was centre of a government. A centre for known and unknown resources. But by the time we exit, there is nothing special about it except that there are some 80 000 people and those who loot people ... It has lost its centre of gravity in the current context."

There have been repeated international calls for an end to the fighting, which has been punctuated by reports of brutal gang-rapes and mass killings of civilians. At least 12 aid workers have been killed.

Abiy acknowledged in a speech to parliament in March that atrocities including rape have occurred and pledged that any member of the Ethiopian army who committed crimes against civilians would be held accountable.

At least 350 000 people are facing famine and 5 million others need immediate food aid, the United Nations has said - the worst global food crisis in a decade.

"If it is required, we can easily enter to Mekelle and we can enter in less than three weeks," Redwan Hussein, spokesperson for the Ethiopian government's task force for Tigray, told reporters.

The Eritreans, who joined the government side after they said that the TPLF had attacked them, had withdrawn from the region, he said.

Eritrea sent troops to Tigray after the Ethiopian military launched an offensive in November in response to attacks on federal government bases by regional forces. Eritrea denied for months that its troops were in Tigray, but later acknowledged their presence while denying they were responsible for abuses.

Eritrea's information minister did not immediately respond to a request for comment on Wednesday.

Eritrea fought a brutal 1998-2000 war with Ethiopia, when the TPLF dominated Ethiopia's central government, and it regards the TPLF as a mortal foe.

'RESPONSE WILL BE HUGE'

The Ethiopian army warned Tigray forces against reorganising, saying its response "will be huge".

"To those who said they might reorganise, they won't pass an inch," Lieutenant General Bacha Debele said. "If they try to provoke, our response will be huge and it will be more than the previous one."

Getachew Reda, spokesperson for the TPLF, told Reuters on Tuesday Tigrayan forces were "100% in control of Mekelle".

On Monday, as reports emerged of the TPLF reaching downtown Mekelle, the federal government issued a statement declaring a unilateral ceasefire with immediate effect.

On Tuesday, Getachew dismissed the ceasefire as a "joke", saying the TPLF would force its enemies "out of our territory". He was not reachable for comment on Wednesday.

Redwan described the ceasefire as a political decision "made for humanitarian cause".

The TPLF, an ethnically based political party that dominated Ethiopia's national politics for nearly three decades, has been battling the central government since early November.

United Nations Secretary General Antonio Guterres said on Monday he hoped a political solution would be possible. The United States said atrocities should end immediately and warned Ethiopia and Eritrea that Washington would be watching closely.

"We will not stand by in the face of the horrors in Tigray," said Robert Godec, acting assistant secretary of state for the State Department's Bureau of African Affairs.