Nigeria's President Muhammadu Buhari needs "further rest" so will extend his month-long medical stay in London, but there is "no cause" for worry over his health, his office said Tuesday.
"During his normal annual checkup, tests showed he needed a longer period of rest, necessitating the president staying longer than originally planned," presidential spokesman Femi Adesina said in a statement.
"President Buhari wishes to reassure Nigerians that there is no cause for worry," he said.
Speaking to reporters, Adesina added: "It makes sense to say that maybe from the results of the tests, further rest had been recommended."
The statement did not say how long the rest would last, and Adesina did not disclose the nature of Buhari's ailment.
"Don't you know that the Hippocratic Oath even forbids a doctor from speaking about the condition of his patient except if the patient authorises it?" he asked.
"It is only the patient himself who can speak about what he is going through. This is the person going through these series of tests and rest and he says no cause to worry, let us believe that."
Buhari's health has been the subject of increasing concern in Nigeria since he extended his stay in London earlier this month just as he was scheduled to return to Abuja.
The presidency has repeatedly insisted he is not seriously ill but awaiting the results of medical tests.
The 74-year-old former army general received treatment for what his office said was a persistent inner ear infection in London in June last year.
The health of Nigeria's president has become a sensitive issue following the 2010 death of president Musa Umaru Yar'Adua from a long-standing, but previously undisclosed, kidney complaint.
Yar'Adua's initial illness and treatment in hospital abroad triggered months of political uncertainty. His deputy, Goodluck Jonathan, took over on Yar'Adua's death.