Ethiopia's army chief of staff and the head of the northern state of Amhara were killed in separate but related attacks when a general tried to seize control of Amhara in an attempted coup, the government said.
Amhara's state president and his adviser were shot dead and the state's attorney general was wounded in the regional capital of Bahir Dar on Saturday, Prime Minister Abiy Ahmed's office said.
In a separate attack the same night, Ethiopia's army chief of staff Seare Mekonnen and a retired general were shot dead in General Seare's home in Addis Ababa by his bodyguard. The two attacks were linked, the statement said, without giving details.
Mr Abiy's office named Amhara state security head General Asamnew Tsige as responsible for the foiled coup, without giving details of his whereabouts.
General Asamnew was released from prison last year after receiving an amnesty for a similar coup attempt, according to media reports.
Mr Abiy took office just over a year ago and embarked on unprecedented reforms in Ethiopia, Africa's second-most populous country and one of its fastest-growing economies.
But the premier's shake-up of the military and intelligence services has earned him powerful enemies, while his government is struggling to rein in powerful figures in Ethiopia's myriad ethnic groups fighting the federal government and each other for greater influence and resources.
US Assistant Secretary of State for African Affairs, Tibor Nagy, said the attacks were probably prompted by disaffection over Mr Abiy's rise to power and his sweeping reforms.
"There are vestiges of the old regime in power. Some of the elites are very unhappy with some of the reforms that... Abiy is taking for a variety of reasons including, I'm sure, some ill-gotten gains," Mr Nagy told reporters in Pretoria, South Africa.
"It's certainly not clear sailing for him (Abiy) from now on. He has an incredible number of issues he has to deal with," Mr Nagy, a former US ambassador to Ethiopia, said.
The shooting in Bahir Dar occurred when the state president - an ally of Abiy - was holding a meeting to decide how to put a stop to the open recruitment of ethnic Amhara militias by General Asamnew, one Addis-based official told Reuters.
General Asamnew had advised the Amhara people to arm themselves in preparation for fighting against other groups, in a video spread on Facebook a week earlier and seen by a Reuters reporter.
Mr Abiy donned military fatigues to announce the attempted coup on state television on Saturday evening.
Residents of Bahir Dar, about 500km northwest of Addis, said there was at least four hours of gunfire on Saturday evening and some roads had been closed off.
"The situation in the Amhara region is currently under full control by the Federal Government in colloboration with the regional government," Mr Abiy said in Sunday's statement.
Brigadier General Tefera Mamo, the head of special forces in Amhara, told state television that "most of the people who attempted the coup have been arrested, although there are a few still at large." He did not give details about General Asamnew.
The US Embassy tweeted it had heard reports of gunfire in Addis Ababa on Saturday night, and some residents told Reuters they heard six shots in a suburb near the country's Bole International Airport around 9:30pm local time.
Since taking power, Mr Abiy has released political prisoners, removed bans on political parties and prosecuted officials accused of gross human rights abuses, but his government is battling ethnic bloodshed once held in check by the state's iron grip.
Now some of Ethiopia's ethnic groups are disputing the boundaries of the country's nine federal states, or arguing that they too should have regional governments, claims that threaten the dominance of other groups.
Amhara is home to Ethiopia's second largest ethnic group of the same name and their native tongue, Amharic, is also the country's official language.