Nigeria's army chief, Lieutenant General Ibrahim Attahiru, died in an air force plane crash on Friday.
He was on an official visit to the northern state of Kaduna, which has had security challenges in recent months.
The air force said in a statement that its plane crashed near Kaduna airport and that it was investigating the cause.
The presidency added that other military officers were also killed in the crash.
President Muhammadu Buhari, in a presidency statement, described the crash as a "mortal blow ... at a time our armed forces are poised to end the security challenges facing the country."
Nigeria, Africa's most populous nation, has had poor air safety record in the past, although it has improved in recent years.
Buhari appointed Attahiru alongside other military chiefs in January after years of mounting criticism over spreading violence by insurgents and armed gangs.
The World Health Organisation (WHO) announced on Thursday the launch of clinical trials for two potential treatments for the Ebola virus in the Democratic Republic of the Congo, with the first patient enrolled in the study.
Rebels in Indonesia's restive easternmost region of Papua on Thursday shot dead an American pilot and set a civilian plane on fire in what a spokesperson for a local separatist group described as a message to the US and Indonesian governments.
Russian forces attacked the Ukrainian capital Kyiv on Thursday, killing at least 10 people and wounding more than 50, as drones and missiles struck residential buildings in what Russia said was a retaliation for recent attacks on its civil infrastructure.
Firefighters were battling to bring a wildfire in the Aude region of southern France under control on Thursday, as the country grappled with the lingering effects of Europe's recent heatwave.
Iran and the US concluded a round of indirect talks on Wednesday with no sign they had made headway toward a lasting peace, focusing instead on issues that they said had been resolved when an interim agreement was announced two weeks ago.
President Donald Trump dedicated a museum honouring Theodore Roosevelt on Wednesday, invoking the Republican president's legacy and linking it to his own vision for America ahead of the country's 250th anniversary.
A Kenyan court charged eight schoolgirls on Wednesday with murder for the deaths of 16 of their fellow students in a dormitory fire at a school in late May.
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Clinical trials begin for two potential Ebola treatments
Blast at Damascus cafe kills nine, wounds 20
Rebels in Indonesia's Papua kill American pilot, burn plane
Russian air strikes kill 10, injure more than 50 in Ukraine's Kyiv
Firefighters battle wildfire in southern France
US, Iran talks conclude in Doha, focused on Strait of Hormuz
Trump honours Theodore Roosevelt in North Dakota, debuts new Air Force One
Kenyan court charges 8 schoolgirls with fellow students' murder
