Nigeria's President Muhammadu Buhari is expected to return to the country on Friday after extended medical leave in the UK, according to a presidency statement.

The 74-year-old, who took power in May 2015, left Abuja on January 19 for treatment in Britain.

He had originally planned to stay 10 days but stayed longer to rest after consulting his doctors, deepening suspicions that his health was far worse than officials are publicly admitting.

"President Buhari expresses appreciation to teeming Nigerians from across the country, and beyond, who had prayed fervently for him, and also sent their good wishes," the presidency said on Thursday, adding he extended his UK stay on doctors' recommendation for further tests and rest.

The statement gave no medical details.

Earlier on Thursday, the presidency had published pictures of a smiling Buhari meeting the Archbishop of Canterbury, Justin Welby, in Abuja House, part of the Nigerian High Commission in London.

No official pictures of Buhari's meetings in London had been posted since February 15.

The government had sought to allay concerns of a void at the helm of Africa's biggest economy and most populous country by stressing that Buhari, who was elected to power on a campaign that vowed to root out corruption, had given Vice President Yemi Osinbajo full powers as acting president during his leave.

Osinbajo, a lawyer, held in Buhari's absence cabinet meetings and finished work on an economic reform plan needed to secure a World Bank loan to help plug a deficit caused by low oil revenues.