Nigerian President Muhammadu Buhari returned to the country on Saturday after more than three months in London for medical treatment, while the government gave no details on what exactly has been ailing him.

Mr Buhari will address the nation in a broadcast on Monday morning, a statement from his office said, referring to the president’s issues merely as a “health challenge”.

The 74-year-old leader also spent seven weeks in London for treatment earlier this year and said he had never been so sick in his life. He spoke of receiving blood transfusions.

Mr Buhari’s long absences have led some in Africa’s most populous nation to call for his replacement and for the military to remind its personnel to remain loyal.

In recent weeks, his office released photos of officials meeting the rail-thin but smiling president in the hope of reassuring people back home.

Mr Buhari did not make any public comment upon his arrival in the capital, Abuja, stepping briskly from the plane and leaving in a motorcade after a salute from the presidential guard.

Nigerians, some of them singing, had lined the road to the airport to welcome him.

Attention to the president’s absence had grown in recent days as he marked 100 days out of the country and as protesters for and against him clashed in the capital.

Nigeria’s ongoing challenges include the deadly Boko Haram insurgency, a weak economy and one of the world’s worst humanitarian crises, with millions malnourished in the north east.

Vice president Yemi Osinbajo has been acting president during Mr Buhari’s time abroad.

Observers have feared that political unrest could erupt in Nigeria, particularly in the predominantly Muslim north, should Mr Buhari not finish his term in office, which ends in 2019. The previous president was a Christian from the south, as is Mr Osinbajo.