as head of Africa's most populous nation on Wednesday, facing heightened pressure to defeat Islamic extremism and boost the oil-dependent economy amid concerns over his health.

The 76-year-old former military dictator read the oath of office from a card as he stood beside his wife, wearing a traditional white robe and sunglasses. He did not make a speech.

Governors also were sworn in across the West African nation, though the ceremony was canceled in Buhari's home state of Katsina in the north because of security concerns.

Buhari won the February election despite serious questions about his health, having spent more than 150 days outside the country for unspecified medical treatment during his first term.

Along with a 25 percent unemployment rate and widespread poverty, Nigeria continues to confront a multitude of security threats that loom over Buhari's second term.

A new extremist faction pledging allegiance to the Islamic State group has made a deadly resurgence in recent months, overrunning military bases in the northeast and raising questions about how much support Nigeria's troops receive from the government. The decade-old Boko Haram insurgency also remains a threat.

Deadly clashes between largely Christian farmers and largely Muslim herders over increasingly scarce land also have wracked central Nigeria. Buhari has faced particularly sharp criticism over the crisis, as many Nigerians worry that he sympathizes with the herders as a fellow ethnic Fulani from the north.

VOA