Former Nigerian President Olusegun Obasanjo has advised the incumbent President Muhammadu Buhari against seeking re-election next year.

Mr Obasanjo supported Mr Buhari's election in 2015 but now he says the president should retire because of age and ill health.

In an open letter, Mr Obasanjo says he is disappointed with Mr Buhari, particularly because of what he calls his poor handling of Nigeria's economy, the largest in Africa.

He adds that the president does not have a good understanding of Nigeria's social and economic dynamics and the country needs a younger leader.

Mr Buhari has not publicly responded to Mr Obasanjo, who wrote a similar letter to then President Goodluck Jonathan urging him not to run in 2015.

Many observers say Mr Buhari, 74, is likely to seek a second term but so far he has not said he would do so.

Mr Obasanjo ruled Nigeria twice, in the 1970s as a military ruler and from 1999 to 2007 as a civilian president, but failed in what many believed was an attempt to change the constitution to enable him seek a third term in office.