Ethiopian Prime Minister Abiy Ahmed says he is launching the “final phase” of the army’s operation in the northern region of Tigray after weeks of fighting.
He said the military would try not to harm civilians in the regional capital Mekelle – a city of 500,000 people – and urged residents to stay at home.
The TPLF party, which controls Mekelle, has vowed to keep fighting.
The UN warns of possible war crimes if the Ethiopian army attacks Mekelle.
Mr Abiy’s announcement comes after a deadline he gave for Tigray fighters to surrender passed on Wednesday.
Hundreds of people have reportedly been killed and thousands have been forced from their homes as Ethiopian forces have seized various towns in Tigray from the TPLF.
However, details of the fighting are hard to confirm because all phone, mobile, and internet communications with the Tigray region have been cut.
Local journalist Daniel Berhane, who is in Mekelle, told the BBC there was no sign yet of an attack, and that shops, cafes, and restaurants in the city were “almost full”.
Three African Union representatives have arrived in the Ethiopian capital Addis Ababa to try to broker talks but Ethiopia has so far rejected all mediation attempts, saying the conflict is an internal matter and Mr Abiy’s government is engaged in a law enforcement mission in Tigray.
The three envoys will not be allowed to travel to Tigray.
He ordered the Ethiopian military to launch an offensive on Mekelle in the “third and final phase” of the federal government’s military campaign against the TPLF.
Mr Abiy said “great care” would be taken to protect civilians and “all efforts” would be made to limit damage to Mekelle.
He urged people in Mekelle and the surrounding areas to disarm, stay at home and keep away from military targets.
Religious and historical sites, institutions, and residential areas would not be targeted, he said.
The leader of the powerful regional party, Debretsion Gebremichael, has said Tigray forces are “ready to die in defense of our right to administer our region”.
The TPLF fighters, drawn mostly from a paramilitary unit and a well-drilled local militia, are thought to number about 250,000. Some analysts fear that the situation could turn into a guerrilla conflict.
One example of how this might come about is the battle for Aksum airport, which according to pro-government sources fell to Ethiopian forces on 11 November.
But 11 days later state media published pictures of the runway apparently strewn with rubble and with trenches dug across it to prevent planes landing, accusing the TPLF of sabotage. Mr Debretsion denied destroying the airport, saying his forces had put up obstacles to stop the Ethiopian army.
Reuters quoted a diplomatic source as saying the TPLF “have mobilised lots of people in Mekelle”. The person added: “They are digging trenches and everyone has an AK-47 [rifle].”