Nigeria's President Muhammadu Buhari, in an address to the nation after returning from London where he spent more than three months for medical treatment, said his government will reinvigorate its campaign against the terrorist insurgency in the country's northeast.

"Terrorists and criminals must be fought and destroyed relentlessly so that the majority of us can live in peace and safety," said Buhari 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. We shall tackle them all."

Buhari returned Saturday to Abuja, though he didn't make any comments upon his arrival to Abuja.

In his address, Buhari,74, did not say what ailment caused him to leave Nigeria in May for the lengthy treatment in Britain. He thanked Nigerians for their prayers.

"I am pleased to be back on home soil among my brothers and sisters," Buhari said.

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.

Earlier this year Buhari spent seven weeks in London for treatment and said he had never been so sick in his life.

In his address, Buhari talked about political divisions, urging that Nigeria must be united. He said that while he was in London he kept in touch with daily events at home.

"Nigerians are robust and lively in discussing their affairs, but I was distressed to notice that some of the comments, especially in the social media have crossed our national red lines by daring to question our collective existence as a nation. This is a step too far," he said.

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.