Ethiopia's army chief was shot dead by his bodyguard just hours after an attempted coup in Amhara state left the regional president and another top adviser dead, a spokeswoman for the prime minister said Sunday.
The spokeswoman Billene Seyoum told journalists a "hit squad" led by Amhara's security chief Asaminew Tsige burst into a meeting on Saturday afternoon, injuring regional president Ambachew Mekonnen and another top official who both died of their wounds.
Later that evening in what appeared a "co-ordinated attack", army chief Seare Mekonnen, and a retired general who was visiting him, were killed by his bodyguard, said Billene.
Since his election last year, Mr Abiy has moved to end political repression by releasing political prisoners, removing bans on political parties and prosecuting officials accused of human rights abuses.
The US state department has warned its staff in Addis Ababa to stay inside, saying it is aware of reports of gunfire in the city on Saturday.
Information is scanty, with reports that the internet has been down, but Prime Minister Abiy, dressed in military fatigues, went on TV to make his announcement.
He said the chief of staff had been shot by "mercenaries".
The Amhara regional officials were killed by their own colleagues during a meeting, he added.
A statement from the prime minister's office added: "The coup attempt in Amhara regional state is against the constitution and is intended to scupper the hard-won peace of the region.
"This illegal attempt should be condemned by all Ethiopians and the federal government has full capacity to overpower this armed group."
Residents in Amhara's regional capital, Bahir Dar, have reported hearing heavy gunfire.
The ruling party in Amhara has accused a former security chief, who was released from jail after Mr Abiy came to power, of being behind the violence.
The homeland of the Amhara ethnic group is the country's second most populous region and has given Ethiopia its state language, Amharic.
Violence between the Amhara and Gumuz ethnic groups left dozens of people dead last month in Amhara and its neighbouring region, Benishangul Gumuz.
Ethnic violence, typically sparked by land disputes, has displaced nearly three million people across Ethiopia.
Another issue the prime minister is having to grapple with is unrest within the military.
In October, he said that hundreds of soldiers who had marched to his office to demand a pay rise, had wanted to kill him.