The African Union-led negotiations had been flagged to start Monday, after a surge in fighting that has triggered alarm in the international community and fears for civilians caught in the crossfire.

But as evening fell, there was no word from any of those involved about whether the talks had started or exactly where they would be held.

Diplomatic pressure has been mounting to bring a halt to the war in Africa's second most populous country that has left millions in need of humanitarian aid, and according to the United States, as many as half a million dead.

Kindeya Gebrehiwot, a spokesman for the rebel authorities in Tigray, announced their delegation's arrival in South Africa in a tweet late Sunday.

"Pressing: immediate cessation of hostilities, unfettered humanitarian access & withdrawal of Eritrean forces. There can't be a military solution!" he added.

Addis Ababa said in a statement its delegation had left for South Africa on Monday morning, adding: "The government of Ethiopia views the talks as an opportunity to peacefully resolve the conflict and consolidate the improvement of the situation on the ground."

But it also said its forces "have continued taking control of major urban centres in the past few days", without identifying them.

Last week, the government vowed to take control of airports and other federal sites in Tigray from the rebels as Ethiopian and Eritrean troops seized towns in the region including the strategic city of Shire, sending civilians fleeing.

Fighting resumed in August, shattering a five-month truce, and has seen the return of the Eritrean army in support of Ethiopian forces and their regional allies.

'Peace will prevail'

Abiy first sent troops into Tigray in November 2020, promising a quick victory over the northern region's dissident leaders, the Tigray People's Liberation Front (TPLF), after what he said were attacks by the group on federal army camps.

The move followed long-running tensions with the TPLF, which had dominated Ethiopia's ruling coalition before Abiy came to power in 2018 and sidelined the party.

In a rare comment on the conflict last week, Abiy -- who won the 2019 Nobel Peace Prize for his rapprochement with Eritrea -- said the war "would end and peace will prevail".

But on Monday head of the rebel region Debretsion Gebremichael issued a defiant statement saying: "The Tigray army has the capacity to defeat our enemies totally.

"Those joint enemy forces that entered Tigray will be buried," he warned. "Our enemies know they can't battle with our troops."

International calls for a ceasefire and a withdrawal of Eritrean troops have grown since the AU failed earlier this month to bring the warring sides to the negotiating table.

A Western official has confirmed that previous secret talks took place organised by the United States in the Seychelles and twice in Djibouti, but gave no further information.

The August return to the battlefield halted desperately-needed aid for Tigray's six million people who lack food, medicine and basic services.

Tigray has also been under a communications blackout for over a year, and independent reporting from the region heavily curtailed.

'End human suffering'

The UN Security Council held a closed-doors meeting Friday to discuss the spiralling conflict, which has raised concerns about the stability of the volatile Horn of Africa region.

The US envoy to the United Nations, Linda Thomas-Greenfield, said after the talks that the scale of fighting and deaths "rival what we're seeing in Ukraine".

"Over two years of conflict, as many as half a million -- half a million -- people have died, and the United States is deeply concerned about the potential for further mass atrocities."

The International Crisis Group (ICG) think tank said while reliable data was scarce, it believed the fighting since August alone may have involved more than half a million combatants and killed tens of thousands of people.

The IGC's Ethiopia senior analyst William Davison said the first objective for mediators was "to try and get the federal and Tigray delegations to agree to a truce despite the momentum towards continued military confrontation".

The AU's mediation team for the South Africa talks was to include Horn of Africa envoy Olusegun Obasanjo, South Africa's former deputy president Phumzile Mlambo-Ngcuka and former Kenyan president Uhuru Kenyatta.

"We count on all parties' political leadership and sense of responsibility to work together to transform commitments into action -- to end human suffering and put Ethiopia on path of reconciliation & reconstruction," EU foreign policy chief Josep Borrell said on Twitter.

Pope Francis admitted he viewed the conflict in the predominantly Christian country with "trepidation".

"May the efforts of the parties for dialogue lead to a genuine path of reconciliation," he tweeted Sunday.