Twelve Nigerian military personnel, mostly high-ranking officers, were killed in a plane crash on last night while six survived, the presidency said in a statement.
President Olusegun Obasanjo rushed home early from an IMF meeting in Singapore following the crash of an air force Dornier 228 aircraft that was conveying senior officers from the capital Abuja to a military meeting in southeastern Cross River state.
Today's statement gave no details on the cause of the crash but said that Mr Obasanjo had ordered an immediate investigation.
"Shocked and saddened, the president has ordered three days of national mourning for the officers, some of whom were in the top echelons of the Nigerian army," the presidency statement said.
Eight major-generals, two brigadier-generals, one lieutenant-colonel and the pilot died in the crash.
The wreck was located in Benue state, 18 nautical miles from its destination, the Obudu Cattle Ranch hotel and conference centre in Cross River.
Mr Obasanjo described the crash as a "monumental national tragedy" and ordered that all flags should fly at half-mast for three days. He also said a national arts festival that was due to open on Monday in the southern state of Bayelsa should be cancelled.
The Nigerian army, which ruled the country almost continuously for three decades until 1999, remains an important player in politics and society.
In a country whose unity is under constant strain from ethnic, religious and regional rivalries, the armed forces are often described as the only truly national institution.
Military air disasters have been relatively frequent in Nigeria over the past two decades. Newspapers on Monday published lists of crashes including one in 1992 that killed 163 military personnel, practically wiping out an entire generation of officers.