His office said Buhari sent a letter on Monday to his counterpart in Guinea
Nigeria's president has said he is recovering from illness and that he expects to return from treatment in London once doctors give the go-ahead, his office said on Tuesday.
Muhammadu Buhari has been on indefinite medical leave in the British capital since May 7 and until Sunday had not been seen, sparking a flurry of rumours he was dying or even dead.
His office at the weekend published a photograph of the 74-year-old head of state meeting a delegation of state governors at Nigeria's diplomatic residence in London.
Now, his office said Buhari sent a letter on Monday to his counterpart in Guinea, Alpha Conde, thanking him for his call last week for national prayers for his recovery.
"Your Excellency (Conde) will be pleased to hear that I am making good progress and as soon as doctors advise I shall return to my duties and continue serving the Nigerian people who elected me and are daily praying for my recovery," it read.
Buhari has been dogged by speculation about his health since mid-2016, when he first went to London for treatment of what the presidency said was a persistent ear infection.
He then spent nearly two months in London in January and February and said on his return in early March that he had "never been so ill".
The exact nature of his condition has not been disclosed and he has appeared faltering and thin.
His main opponents in the 2015 election that brought him to power claimed he had prostate cancer. Buhari denied the claim.
The health of Nigeria's president has been a sensitive issue since the death in office of president Umaru Musa Yar'Adua in 2010 after months of treatment abroad.
Buhari and his advisors have sought to avoid the political paralysis that accompanied Yar'Adua's absence by officially handing over powers to Vice-President Yemi Osinbajo.
But that has also sparked manoeuvrings for power, given long-standing tensions between Nigeria's Muslim-majority north and the Christian-dominated south.
Politicians are already jostling for position to succeed Buhari at the next election in 2019 on the assumption he will not stand for a second term.