The army’s deputy chief of staff accused state troops loyal to the Tigrayan People’s Liberation Front (TPLF) of treason.
The Tigray president said his troops were engaged in clashes along the border with the neighbouring state of Amhara.
The head of the UN has called for a de-escalation of fighting.
What’s the latest update?
More troops are heading to Tigray to provide support for the Northern Command, says Gen Berhanu Jula, deputy chief of staff of the army.
Ethiopia’s Prime Minister Abiy Ahmed insisted in a couple of tweets that this operation would be “limited” and was a last resort after trying “patiently” to resolve differences peacefully with the TPLF for several months.
It comes after Debretsion Gebremicheal, head of the Tigrayan state – and its ruling party the TPLF – said his troops had seized “almost all” of the weaponry at the Command.
The BBC’s Kalkidan Yibeltal in the capital, Addis Ababa, says the crisis is showing no signs of waning.
It is difficult to ascertain the magnitude of the clashes and how many people have been killed.
That’s partly because the internet and phone lines had been cut off in Tigray.
But Mr Abiy has also refrained from giving details, instead promising information would be released once operations were over.
What’s this all about?
Tension has been mounting for some time as power has been shifting – the TPLF was the most powerful partner in Ethiopia’s governing coalition until 2018, when Mr Abiy came to office. Since then, its power has waned.
But one event sparked this fighting – a local election.
In September, the Tigrayan authorities went ahead with an election in defiance of a nationwide postponement that was put in place following the outbreak of coronavirus.
The federal government had described that election for the Tigray parliament as “illegal”.
The vote raised concerns that the TPLF was laying the groundwork for the creation of a breakaway state, with a parliament and government taking office without the blessing of the federal government.
But the TPLF said that it was committed to keeping the region within Ethiopia, but said it would defend “self-rule” and oppose what it called Mr Abiy’s attempt build a strong “unitary” state.
Mr Debrestion told reporters on Monday that the government was going to attack – alleging it was punishment for organising the election.
Mr Abiy then accused the TPLF of attacking the military base and sent in the Ethiopian military.
The federal government has also declared a six-month state of emergency in the state of Tigray, and airspace in the region has been closed.
What are diplomats saying?
The UN Secretary General António Guterres said he was “deeply alarmed” and called for an “immediate de-escalation of fighting”.
Previously, on Wednesday, amid the drama of the US election, US Secretary of State Mike Pompeo also called for an end to any fighting.
“We are saddened by the tragic loss of life and urge immediate action to restore the peace and de-escalate tensions… the protection of civilian safety and security is essential,” he said in a statement.
The African Union, whose headquarters are in the Ethiopian capital, Addis Ababa, is pushing for talks but the government is not willing to negotiate, sources told the Reuters news agency.