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.
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.