Former President John Mahama is in Banjul, the Gambia, to persuade incumbent President Yahya Jammeh to agree "to leave office" after his defeat at the ballot box.
Mr Mahama who is the Co-Mediator left Accra this morning, and is expected to be joined by Liberia's Ellen Sirleaf Johnson and officials of the UN, AU and ECOWAS.
The Gambia, following elections last year, is expected to inaugurate a new president on January 19, but President Jammeh who lost and earlier accepted the results of the elections has now rejected the results.
The Mediators are hoping to strike a deal with President Jammeh to accept the results and pave way for the January 19 inauguration of the new President.
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The delegation headed by Liberian leader Ellen Johnson Sirleaf, a Nobel Peace laureate and the current chairwoman of the Economic Community of West African States (ECOWAS), will include Nigerian President Muhammadu Buhari, Sierra Leone leader Ernest Bai Koroma and former President John Mahama.
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Jammeh seized power in a coup in 1994 when he was an army lieutenant and has ruled ever since, wining four elections, that were criticised by rights monitors, and surviving several coup attempts. International human rights groups accuse him of widespread violations and repression.
He declared Gambia an Islamic Republic last year.
A statement from the opposition coalition read to reporters in Barrow's presence on Monday called for Jammeh to step down immediately.
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However, President Yahya Jammeh has said he will not step down before a Supreme Court decision on disputed elections, a ruling now not expected until May.
In a nationwide TV broadcast, the longstanding ruler also reiterated his concern at "foreign interference".