Ethiopia army launched co-ordinated attacks on all fronts against forces from the northern Tigray region.
The Ethiopian government has not confirmed any fighting.
A senior rebel said the government was using artillery, tanks, jets and drones in an attempt to “reinvade” the region.
Spokesperson for the Ethiopian Prime Minister, Billene Seyoum, said the government had a responsibility to protect its citizens across the country from acts of terrorism but gave no further details.