ABUJA, Nigeria — Nigeria’s President, Muhammadu Buhari, said his government will step up its campaign against Islamic extremist rebels but made no mention of his health as he spoke to the nation for the first time Monday, after more than three months of medical treatment in London.

‘‘Terrorists and criminals must be fought and destroyed relentlessly so that the majority of us can live in peace and safety,’’ Buhari said in a televised speech on Monday.

‘‘Therefore we are going to reinforce and reinvigorate the fight not only against elements of Boko Haram, which are attempting a new series of attacks on soft targets, kidnappings, farmers versus herdsmen clashes, in addition to ethnic violence fueled by political mischief makers,’’ he said. “We shall tackle them all.’’

Buhari, 74, did not say what illness caused him to leave Nigeria in May for the lengthy treatment in Britain. Earlier this year he spent seven weeks in London for treatment and said he had never been so sick in his life.

The government of Africa’s most populous nation has never said what exactly has been ailing Buhari, and his long absences have led some to call for his replacement and for the military to remind its personnel to remain loyal.

ASSOCIATED PRESS