Ethiopian Prime Minister Abiy Ahmed says he is launching the "final phase" of the army's operation in Tigray with an attack on the regional capital, Mekelle.
He said the military would try not to harm civilians and urged people in the city to stay at home.
It comes after a deadline he gave for Tigray fighters to surrender passed on Wednesday.
The TPLF party, which controls Tigray, has vowed to keep fighting.
Hundreds of people have reportedly been killed and thousands have been forced from their homes.
However, details of the fighting are hard to confirm because all phone, mobile and internet communications with the Tigray region have been cut.