The authorities in Ethiopia’s Amhara region say they will go on the offensive against forces from neighbouring Tigray.
The Tigrayan rebels have recaptured much of Tigray in recent weeks, with federal forces and their allies withdrawing from key areas.
A spokesman for the Amhara regional government said patience had run out.
Retired members of the security forces have been called up and volunteer militias asked to mobilise.
Tigrayan forces are pushing towards territory seized by the Amhara earlier on in the civil war. Both groups see it as rightfully theirs.
With passions so high, this could lead to a new and even more bloody phase of the conflict.
Ethiopia was previously the most stable country in the Horn of Africa and a key ally of the West in the war on terror.
Now it risks not only tearing itself apart but increasing instability across one of the most fragile corners of the continent.