The Tigray People's Liberation Front, which is refusing to surrender its rule of the northern region, said its forces were digging trenches and standing firm.

Prime Minister Abiy Ahmed's federal troops have taken a string of towns during aerial bombardments and ground fighting, and are now aiming for Mekelle, a highland city of about 500,000 people where the rebels are based.

Ethiopian forces are claiming they've taken the town of Idaga Hamus, 97 km from Mekelle.

The war has killed hundreds, possibly thousands, sent more than 30,000 refugees into neighbouring Sudan, and seen rockets fired by rebels into neighbouring Amhara region and across the border into the nation of Eritrea.

Countries around Africa and Europe have urged a truce, but Abiy has so far rebuffed that.

"The next phases are the decisive part of the operation, which is to encircle Mekelle using tanks," military spokesman Colonel Dejene Tsegaye told Ethiopia Broadcasting Corporation.

"We want to send a message to the public in Mekelle to save yourselves from any artillery attacks and free yourselves from the junta ... After that, there will be no mercy."

Rebel leader Debretsion Gebremichael claims his forces were resisting a push from south of Mekelle while also fighting near the northern town of Adigrat after it had fallen to federal troops.

Abiy accuses the Tigrayan leaders of revolting against central authority and attacking federal troops in the town of Dansha on November 4. The rebels say his government has marginalised and persecuted Tigrayans since taking office two years ago.

The prime minister drew plaudits for opening up Ethiopia's closed economy and repressive political system after taking office. He won a Nobel Peace Prize last year for a peace agreement with Eritrea.

International rights groups say his government has carried out mass arrests after outbreaks of violence and detained journalists this year. The Nobel Peace Prize committee urged peace after his offensive against the Tigrayan leaders began.

Aid agencies fear a humanitarian disaster in a region where hundreds of thousands relied on food aid and were displaced even before fighting began.

Australian Associated Press