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

Buhari will address the nation in a broadcast 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 this year and said he had never been so sick in his life. He spoke of receiving blood transfusions.

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 with the rail-thin but smiling president in the hope of reassuring people back home.

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 Buhari 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 northeast.

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

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

This isn’t the first time Nigeria has faced a leader’s long absences. When former President Musa Yar’Adua was ill abroad for months before coming home to die in 2009, northerners blocked his southern Vice President Goodluck Jonathan from assuming power, creating a months-long political paralysis.

Jonathan was eventually confirmed, but his subsequent successful run for election angered many Muslims, breaking an unwritten agreement that power rotate between northerners and southerners.

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