The offensive marks the most serious escalation in the region since a peace agreement ended the previous conflict. Both sides are now accusing each other of triggering a return to open hostilities.
The Ethiopian government has warned of 'destruction' as armed groups press their advance. The Tigray People's Liberation Front (TPLF) and its allies have reportedly mobilised fighters and pushed beyond Tigray's borders into areas that saw some of the worst violence during the earlier war.
The new rebel coalition, formed in recent weeks, has explicitly stated its goal of overthrowing Abiy's administration, a far broader objective than the territorial disputes that characterised the previous conflict. Analysts say this could make a negotiated settlement more difficult.
The resumption of fighting has sparked alarm among international observers, who fear a repeat of the humanitarian catastrophe that accompanied the 2020-2022 war, including widespread displacement and reports of atrocities by all sides. The United Nations and African Union have called for an immediate ceasefire, but no formal diplomatic process has been announced.