There are concerns about a leadership vacuum as one of Africa's largest economies struggles to pulls itself out of recession, correspondents say.

Information Minister Lai Mohammed said Mr Buhari "needed to rest".

In March, Mr Buhari returned from seven weeks of sick leave in the UK where he was treated for an undisclosed illness.

When he returned home he said he had never been so ill in his life.

To calm fears, the president's wife Aisha Buhari tweeted on Tuesday that her husband was not as sick as is being perceived.

But as he has not been seen in public for more than a week, Nigerians are continuing to discuss the issue.

In her tweets, Mrs Buhari said that the president was continuing to "carry out his responsibilities" and has been meeting with ministers.