Nigerian President Muhammadu Buhari on Tuesday replaced the country's top military commanders, his office said, after months of pressure over his response to worsening security.

Buhari, a former army general first elected in 2015, came to power vowing to tackle Nigeria's long jihadist insurgency in the northeast.

"President Muhammadu Buhari has accepted the immediate resignation of the service chiefs," a presidency statement said, referring to the chief of defence staff and the chiefs of the army, navy and air force.

The statement did not give any reasons for the overhaul of the country's top ranked commanders.

For more than a decade, Nigeria's military has battled an insurgency by the radical Islamist group Boko Haram that has devastated the northeast, killing at least 36,000 people and displacing more than two million.

The Islamic State in West Africa (ISWAP), a rival jihadist group that split from Boko Haram in 2016 and is backed by the Islamic State, has in recent years stepped up deadly attacks on military bases.

The country is also struggling with a widening conflict over land and resources between mainly Christian farmers and nomadic mostly Muslim herders in which thousands have died over the past years.