Article published on the 2008-08-21 Latest update 2008-08-21 13:49 TU
Just one top officer appointed by former president Olusegun Obasanjo has stayed. The three who were fired were seen as Obasanjo loyalists.
Yar'Adua, who took power last year, is the first democratically elected leader to succeed another in Nigeria, which has been plagued by military coups since independence.
Obasanjo, a former military ruler, chose Yar'Adua as the ruling party's candidate for the 2007 presidential election after his failed attempt at seeking a third term.
Although they come from the same People's Democratic Party (PDP) as Obasanjo, there has been a series of corruption allegations against members of the previous administration.
The PDP said there has been speculation that the government is not happy with the way the military has handled the problems in the turbulent oil-producing Niger Delta region.
Attacks by militants have cut Nigeria's oil output by about 20% - something Yar'Adua promised to address when he took power.Some military officials have been accused of being involved in the widespread theft of oil.