Ethiopia’s Prime Minister, Abiy Ahmed, has appealed to citizens to take up arms to block advancing rebel fighters.
He made the call on Facebook after the rebels, from the northern Tigray state, reportedly seized control of more towns in neighbouring Amhara.
The US has called for a ceasefire in a year-long conflict that has created a humanitarian crisis.
The Tigray People’s Liberation Front (TPLF) says its aim is to break a siege of the northern region.
In his statement on Sunday, Mr Abiy said the rebels’ advance was “pushing the country to its demise”.
He urged citizens to “organise and march through [any] legal manner with every weapon and power… to prevent, reverse and bury the terrorist TPLF”, according to a translation on the Addis Standard news site.
His comments coincided with the rebels reportedly capturing the strategic towns of Dessie and Kombolcha in Amhara state, just over 300km (186 miles) north of the capital, Addis Ababa.
In a tweet on Monday, the Ethiopian government said the rebels “summarily executed more than 100 youth residents” in the Kombolcha area. The TPLF has not commented and there has been no confirmation of the killings.
It has been difficult to verify claims made by both sides in the conflict because communications have been restricted.
The US Secretary of State Antony Blinken has expressed alarm over the escalating conflict. In a tweet, he said continued fighting only prolonged the dire humanitarian crisis in northern Ethiopia.
Thousands of people have been killed in the war, millions displaced and hundreds of thousands are facing famine conditions, the UN says.
Both sides of the conflict have also been accused of committing atrocities, but they both deny the allegations.
The war broke out on 4 November last year, when Ethiopian Prime Minister Abiy ordered a military offensive against regional forces in Tigray.
He said he did so in response to an attack on a military base housing government troops there.
The escalation came after months of feuding between Mr Abiy’s government and leaders of the TPLF, which was the dominant political party in Tigray.
The authorities later labelled the TPLF a terrorist organisation and ruled out any peace talks with them.
The federal government’s renewed ground offensive in recent weeks, including using airstrikes has failed to halt the rebels’ territorial gains.