Ethiopia's prime minister has given Tigrayan forces 72 hours to surrender before the military begins an offensive on the regional capital.
"We urge you to surrender peacefully within 72 hours, recognising that you are at the point of no return," he said.
The prime minister also said that "all the necessary precautionary measures have been taken to ensure that civilians are not harmed".
His statement came after a military spokesman appeared to threaten that "no mercy" would be shown to civilians remaining in the Tigrayan capital Mekelle.
Colonel Dejene Tsegaye told state-run Ethiopia Broadcasting Corporation that the military planned to encircle the city with tanks and added: "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."
Human rights campaigners warned any such action could violate international law, while former US national security adviser Susan Rice commented: "In other words, war crimes”.
"Treating a whole city as a military target would not only unlawful, it could also be considered a form of collective punishment," said Laetitia Bader, Horn of Africa director for Human Rights Watch.
A spokeswoman from the prime minister's office later claimed that Colonel Dejene meant there would be "no mercy" for the TPLF leadership rather than for civilians.
The TPLF said its forces were digging trenches and resisting a push by government forces from the south. leader Debretsion Gebremichael told Reuters news agency: "Encircling Mekelle is their plan but yet they couldn't... They (are) sending waves after waves but to no avail."
More than 30,000 refugees have fled into neighbouring Sudan since the conflict erupted on 4 November.
Mr Abiy has so far rejected talks with the TPLF and pressed on with the offensive after describing the conflict as a "law enforcement operation".
On Sunday he urged the people of Mekelle to stand with federal troops in "bringing this treasonous group" to justice and added: "all that the clique is left with is the fort that they have set up in Mekelle and empty pride".
The rebels say his government has marginalised Tigrayans since taking office two years ago, removing them from senior roles in government and the military and detaining many on rights abuse and corruption charges.
Earlier this week the UN warned that a “full-scale humanitarian crisis is unfolding” as about 2 million people in Tigray urgently need help due to shortages of food, fuel, medical and other supplies.
Additional reporting by agencies