Prime Minister Abiy Ahmed vowed Wednesday to repel attacks by Ethiopia's "enemies" after rebels in Tigray launched a fresh offensive, casting doubt on a government-declared ceasefire in the war-torn region.

Tigrayan forces this week claimed a series of battlefield gains in a renewed assault that comes two weeks after the federal government declared a unilateral ceasefire in the face of rebel advances.

Abiy, a winner of the Nobel Prize, said the government chose peace at a "cost" in the hope it would quell fighting, allow farmers to plant harvest, and facilitate aid into the stricken region.

But Ethiopia's enemies were "unable to rest without conflict" and posed a threat that must be curbed, he said, signalling a possible end to the June 28 ceasefire.

Rebel fighters in Tigray took over the regional capital Mekele in late June after eight months of conflict Rebel fighters in Tigray took over the regional capital Mekele in late June after eight months of conflict Photo: AFP / Yasuyoshi Chiba

"We will defend and repel these attacks by our internal and external enemies, while working to speed up humanitarian efforts," Abiy said in a statement posted on Twitter.

Abiy, who won by a landslide in June elections to secure a five-year term, urged Ethiopians to stand behind the army in defence of the nation and resist "outside pressure and internal provocation".

The 44-year-old leader sent troops into Tigray last November after accusing the region's once-dominant ruling party, the Tigray People's Liberation Front (TPLF), of orchestrating attacks on Ethiopian military bases.

Map showing the Tigray region of Ethiopia Map showing the Tigray region of Ethiopia Photo: AFP / Aude GENET

Abiy quickly declared victory but in June the rebels recaptured Mekele, the regional capital, in a stunning reversal in an eight-month conflict that has killed thousands of people and pushed the region to the brink of famine.

Abiy and Ethiopian officials characterised their troop withdrawal from Mekele as a strategic move.

But the rebels, rebranded as the Tigray Defence Forces (TDF), described it as a major victory and decried the ceasefire as a "joke".

They later accepted it "in principle" while demanding the complete withdrawal from Tigray of Eritrean forces and militia from the neighbouring Amhara region backing federal troops in the conflict.

But on Tuesday, the TDF announced a new phase of the war, claiming it had seized Alamata, the main town in the region's south, and pushed westward toward fertile farmland long contested by ethnic Amharas.

The Tigray Defence Forces (TDF) recaptured the regional capital Mekele at the end of June The Tigray Defence Forces (TDF) recaptured the regional capital Mekele at the end of June Photo: AFP / Yasuyoshi Chiba

The United States has raised concerns about ethnic cleansing in western Tigray, where Amharan forces have been accused of expelling thousands of people from land they claim rightfully belongs to them.

A spokesman for the Tigrayan forces told AFP the rebels would "liberate every square inch of Tigray".

The rebel claims could not be independently confirmed as communications were largely down in the area.

But the United Nations and humanitarian sources reported fighting around several towns in western Tigray, and confirmed that clashes between rival forces in a refugee camp had sent people fleeing.

The US ambassador to the UN, Linda Thomas-Greenfield, voiced concern on Wednesday at reports of expanding conflict in Tigray, saying that "continued fighting will only lead to needless suffering and death".

"All parties to the conflict should immediately agree to a negotiated ceasefire and political dialogue," the ambassador said in a statement on Twitter.

The war has been characterised by atrocities against civilians, including grisly massacres and rampant sexual violence, including allegedly by Eritrean forces.

The UN Human Rights Council on Tuesday called for an immediate end to all violations in Tigray and for Eritrean troops to quickly withdraw in a verifiable manner.

In his statement Wednesday, Abiy accused Tigrayan forces of conscripting and drugging child soldiers, and claimed the army had a "moral responsibility" to intervene on their behalf.

The war has badly damaged Abiy's standing as a peacemaker, and strained Ethiopia's ties with traditional allies that a few years ago were praising the young leader's committment to democratic reform in Africa's second-most populous nation.

Western powers have demanded the ceasefire be accompanied by unfettered aid access and the withdrawal of Eritrean troops, warning of possible sanctions should these conditions not be met.

The World Food Programme said this week that it had reached Mekele with food supplies but far more was needed to address the massive need in a region where the United Nations says famine conditions are already present.