When President Buhari returned home in March, details of his medical condition were not disclosed, but he said would need more rest and health tests.

Many Nigerians were skeptical about his capacity to continue to work at the high pace required of his office.

It was therefore not shocking to many when the 74-year-old went missing at official events last week.

Now, civil society leaders have said in an open letter that President Muhammadu Buharo should take medical leave immediately.

In a letter titled "President Buhari should take medical leave immediately", a group of political activists noted his absence from the last two weekly cabinet meetings and speculation about his ability to run Africa's most populous nation and biggest economy.

"We are compelled to advise him to heed the advice of his personal physicians by taking a rest to attend to his health without any further delay," they said in the letter dated May 1.

The letter was signed by 13 civil society leaders including human rights lawyer Femi Falana and Jibrin Ibrahim, an academic at a think-tank based in the capital Abuja.

One of Buhari's spokesmen, Femi Adesina, declined to comment on the letter.

Garba Shehu, another presidential spokesman, last week said Buhari received daily briefings on activities of government and met his vice president regularly.

He also said Buhari was spending most of his time in his private residence, which is equipped as an office, adding that he had gone through the worst period of his recovery in London.