Where asked about the President's current state of health, Lai Mohammed said Buhari is in competent hands.
Lai Mohammed was asked about the latest situation after the weekly cabinet meeting of senior ministers in the capital, Abuja.
"Once again, I want to make myself very, very clear. On the issue of Mr President, he is in very competent hands and there is no cause for alarm," he told reporters.
Buhari, 74, has spent a large part of this year in London receiving treatment for an unspecified medical condition.
He left Abuja on May 7 for what his office said was a "follow-up medical consultation" in the British capital and has not been seen or heard from since.
His wife, Aisha, has left Abuja to be with her husband. She previously played down rumours he was too ill to rule but said nothing on her departure from the capital on Tuesday.
Buhari, a former military ruler, has been dogged by speculation about his health even before the election win in 2015 that brought him to power.
During that election campaign, the then-ruling party claimed he was seriously ill with prostate cancer and even put out advertisements insinuating he would die in office.
Buhari rejected the claims as unfounded and designed to portray him as unfit for office.
He first sought treatment in London in June last year for what his aides said was a persistent inner ear infection, returning six months later for check-ups that was later said to be "a long period of treatment".
Buhari himself admitted on his return to Abuja in March that he had undergone blood transfusions and a barrage of tests.
"I couldn't recall being so sick since I was a young man," he added.
He made few public appearances until May 7, when he was seen looking painfully thin at a photocall with 82 schoolgirls released after spending over three years as Boko Haram hostages.