Nigeria's President Bola Tinubu has declared three days of national mourning after a military aircraft crashed in southern Ondo State on Monday morning, killing 25 people.

The Nigerian Air Force confirmed on Tuesday that five crew members and 20 passengers died in the crash. The figure revised an earlier statement by Aviation Minister Festus Keyamo, who had said 32 people were killed.

Authorities say the aircraft, identified as an ATR-42 with tail number NGR 931, was on a routine mission from Benin City to Lagos when it came down in a swamp near a naval base in Igbokoda, Ondo State. The plane was found submerged under water after rescue teams reached the site.

The Nigerian Air Force released the names of those killed, confirming that 19 of those on board were military personnel.

Keyamo said earlier that efforts to contact the aircraft by radio had failed before it was confirmed the plane had crashed. He added that search and rescue operations were ongoing.

A local resident reported that clothes and shoes had been found at the crash site. Smoke was seen rising from the area.

The ATR-42 aircraft has a passenger capacity of between 30 and 50, according to its manufacturers.

It is the deadliest crash involving a Nigerian military aircraft since 1992.

Tinubu offered his condolences to the families of those killed and ordered a full investigation into the incident. He also directed that national flags be flown at half-mast across the country.

"This is a painful moment for our Armed Forces and for the entire nation," Tinubu said.

"Our Air Force personnel put their lives on the line every day to secure Nigeria. Their sacrifice will never be forgotten," he added.