The Ethiopian military will use tanks to encircle Mekelle, the capital of the northern Tigray region, and is warning civilians it may also use artillery on the city, state media reported on Sunday.
It comes as the Ethiopian government rebuffed an African effort to mediate, saying its troops had seized another town in their march towards the rebel-held capital.
"The next phases are the decisive part of the operation, which is to encircle Mekelle using tanks, finishing the battle on mountainous areas and advancing to the fields,” Col Dejene Tsegaye, a military spokesman, told the state-run Ethiopia Broadcasting Corporation.
The conflict erupted on November 4 after what the government described as a surprise attack on federal troops by forces from the Tigray People's Liberation Front (TPLF), the political party that controls Tigray.
Information on the fighting is extremely patchy and claims by all sides are difficult to verify because phone and internet links to the region have been down since the conflict began.
Both sides say federal forces have taken Adigrat, 116 kilometres north of Mekelle.
Col Dejene said civilians in Mekelle, which has a population of half a million, should be aware of the danger.
“So far, we were only attacking targets which the junta fighters were stationed (at) but in the case of Mekelle it might be different,” he said, referring to the TPLF.
“We want to send a message to the public in Mekelle to save themselves from any artillery attacks and free themselves from the junta. The junta is now shielding itself within the public and the public must isolate itself from the junta.”
“After that, there will no mercy.”
TPLF forces were not immediately available for comment.
The government says Tigrayan forces are bulldozing roads and destroyed bridges to hold up the advance on Mekelle. Although the TPLF denies destroying infrastructure, they have promised "hell" for their advancing enemies.
Hundreds, possibly thousands, have died and more than 30,000 refugees have fled to Sudan since the conflict erupted. The fighting has spread beyond Tigray, whose forces have fired rockets at the neighbouring Amhara region and the nation of Eritrea, spurring concern of a wider war and the splintering of multi-ethnic Ethiopia.
Eritrea denies TPLF allegations it sent soldiers over the border to back Prime Minister Ahmed Abiy's offensive against the Tigrayan forces, who are also an old foe of Eritrea.
On Friday, the African Union bloc appointed former presidents Joaquim Chissano of Mozambique, Ellen Johnson Sirleaf of Liberia and Kgalema Motlanthe of South Africa as special envoys to seek a ceasefire and mediation talks.
Mr Abiy, who won the Nobel Peace Prize last year for a peace pact with Eritrea, aims to capture TPLF leaders before talking.
"News circulating that the envoys will be travelling to Ethiopia to mediate between the Federal Government and TPLF’s criminal element is fake," the government tweeted on Saturday.
Mr Abiy accuses the Tigrayan leaders of revolting against central authority and attacking federal troops in the town of Dansha. The rebels say Mr Abiy's government has marginalised and persecuted Tigrayans since taking office two years ago.
Mr Abiy denies that, saying he is seeking only to restore law and order and preserve the unity of Ethiopia and its 115 million people.
On Saturday, the government released a video of a roomful of soldiers of Tigrayan ethnicity holding a meeting about the conflict. Soldiers listen to a speaker and take turns to speak to the camera.
Billene Seyoum, the spokeswoman for the prime minister's office, said all the servicemen were active duty Tigrayan soldiers and that the meeting was part of a campaign to ensure that the military was not aligned to any political party or ethnic group.
In the video, one soldier says the military has been like a family member to the Tigrayan people. Several condemn attacks on the military. One says: "Dividing the military along ethnic lines in such a way is completely unacceptable".
Aid workers say the conflict is creating a humanitarian crisis in Tigray, where many among the more than five million population were already displaced and relying on food aid even before the conflict.
Satellite images from US-based space company Maxar Technologies showed destroyed buildings lining the main road near Dansha airport, where the government says there was a November 4 surprise attack on federal troops.
The TPLF is popular in its home region and dominated national politics from 1991 until Mr Abiy took office. Mr Abiy's parents are from the larger Oromo and Amhara ethnic groups.
"We will do all that is necessary to ensure stability prevails in the Tigray region and that our citizens are free from harm and want," the prime minister tweeted on Saturday.
Over the border with Sudan, the UN is planning for the possible arrival of 200,000 refugees.
"The situation is very dire," said Jens Hesemann, emergency response coordinator for the UN refugee agency UNHCR, from the Hamdayet crossing point, appealing for urgent donor aid.