Botswana's President Ian Khama has reiterated his call for his Zimbabwean counterpart Robert Mugabe to step down in an open letter released on Tuesday.

In the letter, Khama, a Mugabe critic, calls on the 93-year-old to do the "honourable thing" and voluntarily make way for a new leader for the Southern African country.

"The people of Zimbabwe have for a long time been subjected to untold suffering as a result of poor governance under your leadership. It is therefore my conviction that by vacating the Presidency, this will usher in a new political dispensation that will pave the way for the much needed socio-economic recovery in Zimbabwe," Khama writes.

"I therefore reiterate my appeal to you to honourably step aside, if you really care for them, as you profess, and if you cannot find it in you to do so, then as a Christian do so in the spirit of our Lord in order to usher in a new period going forward of unity, peace and prosperity for Zimbabweans and allow your country to be the economic powerhouse it is capable of being."

The letter was released ahead of the Zimbabwean Parliament initiating impeachment proceedings against Mugabe.