Cameroon's 92-year-old President Paul Biya, the world's oldest head of state, has overhauled the military's top ranks in what analysts say is an effort to ensure the armed forces back his bid for an eighth term after a public outcry.

The personnel moves, announced late on Tuesday in a series of presidential decrees, affect nearly all branches of the armed forces.

They include the appointment of new chiefs of staff for the infantry, air force and navy as well as the promotion of eight brigadier generals to the rank of major general.

One of the promoted generals is the coordinator of the elite Rapid Intervention Battalion (BIR), a special forces unit often deployed in counter-terrorism operations and seen as central to Biya's security apparatus. The decrees also named a new special presidential military adviser.

The decrees were published two days after Biya, in power since 1982, announced he would run for his eighth term in office in Cameroon's presidential election scheduled for October 12. The seven-year term could keep him in office until he is nearly 100.

The announcement prompted an unprecedented public outcry in the press and on social media in Cameroon, where Biya's age and long absences have raised questions about his fitness to rule.

The government has said Biya is in good health and dismissed any suggestions otherwise.

The decrees concerning the armed forces reflect "a strategy by President Biya and his collaborators to consolidate power by building a fortress of loyal army generals around him" that can suppress any protest to his continued rule, said Anthony Antem, peace and security analyst at the Nkafu Policy Institute in Yaounde.

Celestin Delanga, researcher at the Institute for Security Studies (ISS), said the decrees "come in a unique political and security context."

Biya wants to ensure stability during and after the election, and "additional trusted personnel are needed" for that, he said.

The cocoa- and oil-producing Central African nation also faces a host of serious security challenges, notably a conflict with Anglophone separatists and threats from Nigeria-based Islamist fighters in the north.

The government gave no explanation for the overhaul.

The last significant military shake-up in Cameroon came just last year, shortly after Biya returned in October from his latest extended stay abroad which revived speculation about his health.