Nigerian President Muhammadu Buhari has returned to the country on Saturday after more than three months in the UK for medical treatment for an undisclosed "health challenge".

The 74-year-old left for London on May 7.

His prolonged absence caused tensions back home where calls have grown for him to either return or resign.

"President Buhari is expected to speak to Nigerians in a broadcast by 7am on Monday," his office said.

In recent weeks, the office has been releasing photos of officials meeting with the rail-thin but smiling president in the hope of reassuring people back home.

READ MORE - Nigerians tell absent Buhari: 'Resume or resign'

This was Buhari's second break for medical leave in London this year after a first that began in January and lasted nearly two months.

He reduced his working day to a few hours after returning to Nigeria from his first stint of medical leave on March 10, diplomats and government sources said.

His main opponents in the 2015 election that brought him to power claimed he had prostate cancer. He has denied that claim.

Buhari's four-year term ends in 2019.

Challenges facing Nigeria

Nigeria's ongoing challenges include deadly Boko Haram attacks, a weak economy and one of the world's worst humanitarian crises, with millions malnourished in the northeast.

With Buhari away, Nigeria has been led by acting president and Vice President Yemi Osinbajo, who met the president in London in mid-July and announced that Buhari would return to Nigeria "very shortly".

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

The health of Nigeria's leaders has been a sensitive issue since the 2010 death in office of President Umaru Musa Yar'Adua after months of treatment abroad.