Africa's most populous country goes to the polls on February 25, but has been plunged into crisis in the runup to the vote with dire shortages of cash and fuel.

The African Union Election Observation Mission (AUEOM) is being headed by former Kenyan president Uhuru Kenyatta, who helped broker a deal to end two years of war in northern Ethiopia and is mediating in the conflict in eastern Democratic Republic of Congo.

The AU said the objectives of the mission were to provide an "accurate and impartial assessment" of the electoral process, offer recommendations for any improvement in future polls and to demonstrate AU support "towards consolidation of democracy, peace, stability and development in Nigeria".

Nearly 100 million people will vote to chose the successor to President Muhammadu Buhari, who steps down after two terms as Nigeria struggles with widespread insecurity and economic malaise.