Nigeria's President Muhammadu Buhari has addressed the nation after spending more than three months in London for medical treatment.

In a televised address, Buhari on Monday thanked Nigerians for their prayers and said he was pleased to be "back on home soil among my brothers and sisters".

Buhari returned to Nigeria Saturday after a long absence that has led some to call for his replacement and for the military to remind its personnel to remain loyal.

The government of Africa's most populous nation has never said what exactly is ailing the 74-year-old leader.

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

Nigeria's ongoing challenges include the deadly Boko Haram insurgency and a weak economy.

In Monday's speech, broadcast two days after his return on Saturday, Buhari said he followed events in Nigeria on a daily basis during his absence and was concerned by discussions over the possible dissolution of the country.

"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," said Buhari.

Buhari also said Nigeria's security agencies should not let successes achieved against Boko Haram over the last 18 months "be a sign to relax", adding that Nigeria would "reinforce and reinvigorate" the battle against the fighters.

"His speech was very big on unity and he did mention security problems," said Al Jazeera's Catherine Soi, reporting from Abuja. "He did mention how distressed he was about some comments on social media but people were expecting him to give more details about his status, about what has been ailing him.

"People wanted to hear more about that but he did not mention anything to do with that, so that's a disappointment."

Soi added, however, that many Nigerians had noted Buhari looked in better shape than when he had returned in March.

"When he returned in March, he said he would take things easy. We saw him miss some cabinet sessions and Friday prayers - but he does look stronger to when he returned in March," she said. "People are hopeful that he's gotten better. Many Nigerians are very relieved that he is back and looking much stronger.

"People lined the streets when he arrived."