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.
Iran described US proposals to end a month of war in the Middle East as "unrealistic, illogical and excessive" on Monday and unleashed more missiles on Israel, as oil prices rose further after Yemen's Houthis entered the conflict.
President Donald Trump said on Monday the United States was in talks with a "more reasonable regime" to end the war in Iran but repeated his warning to Tehran to open the Strait of Hormuz or risk US attacks on its oil wells and power plants.
An Israeli air strike has killed at least three people in the Gaza Strip on Monday, local health officials said, in the latest round of violence since a US-brokered ceasefire took effect more than five months ago.
Spain has closed its airspace to US planes involved in attacks on Iran, a step beyond its previous denial of use of jointly-operated military bases, Spanish newspaper El Pais reported on Monday, citing military sources.
A United Nations peacekeeper has been killed in southern Lebanon overnight, prompting condemnations on Monday after a weekend in which Lebanese journalists and medics were killed in Israeli strikes.
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