International pressure mounted Tuesday on the warring parties in northern Ethiopia to cease fighting and protect civilians in Tigray, whose leaders Prime Minister Abiy Ahmed has given a deadline to surrender.

The UN Security Council will convene its first meeting Tuesday on Tigray, where forces loyal to the region's ruling party have been battling Ethiopian soldiers for nearly three weeks, sparking a refugee exodus, civilian deaths and fears of broader instability in the Horn of Africa.

Abiy, last year's Nobel Peace Prize winner, ordered troops, tanks and warplanes into Tigray on November 4 in response to an alleged attack on federal military camps by the Tigray People's Liberation Front (TPLF).

The army says it has encircled Mekele, the regional capital, ahead of a threatened all-out assault. The prime minister, who has resisted calls for mediation and insists the conflict has reached a decisive final stage, on Sunday gave the TPLF 72 hours to surrender -- an ultimatum rejected by the dissident region's leader, who says his people are "ready to die" for the homeland.

The prospect of a devastating bombardment of Mekele, a city of half a million people and a refuge for many more fleeing the conflict, has alarmed rights groups who warn such an attack on a civilian centre could contravene international law.

"As Ethiopian federal troops begin preparations to encircle Mekele, Amnesty International reminds all parties that deliberately attacking civilians and civilian objects is prohibited under international humanitarian law, and constitutes war crimes," said Deprose Muchena, head of Amnesty International's east and southern Africa office. "Indiscriminate and disproportionate attacks are also prohibited."

Amnesty appealed to both sides of the conflict not to use heavy artillery in crowded areas, and not to use human shields or place military camps near civilian sites that could be targeted. The Security Council meeting will not be open to the public, and is being held at the request of South Africa, whose head of state Cyril Ramaphosa is the current chairman of the African Union (AU).

'End this conflict'

The 55-member AU, headquartered in the Ethiopian capital Addis Ababa, has dispatched three former African presidents as special envoys to try and broker talks on the Tigray crisis, joining an international chorus calling for an urgent resolution.

But on Monday a spokesman for an Ethiopian committee handling the conflict, Redwan Hussein, again ruled out negotiations with the TPLF but said the government would meet the AU envoys "as a matter of respect".

The US National Security Council on Monday called for mediation and extended its support to Ramaphosa and the AU diplomatic effort "to end this tragic conflict now". France also condemned the violence, with foreign ministry spokeswoman Agnes von der Muhll calling for urgent measures to protect civilians.

The fighting has already driven more than 40,000 people into Sudan and reportedly killed hundreds and forced many more to flee within Tigray, though the true extent of death and displacement is not yet known.

A communications blackout and restrictions on reporting in the region have made claims from both sides difficult to verify. The TPLF on Monday said it routed an army battalion and claimed responsibility for a rocket attack on Bahir Dar, the capital of Amhara region to the south of Tigray, whose local forces are fighting alongside Ethiopian troops.

The government said Tuesday that "a large number of Tigray militia and special forces are surrendering" following the issuance of Abiy's 72-hour ultimatum. Long-running tensions between Addis Ababa and the TPLF, which dominated Ethiopia for nearly three decades before Abiy came to power in 2018, boiled over in September when Tigray proceeded with regional elections in defiance of the federal government, which declared the vote illegal.