Mr Jammeh has been given until noon on Friday to leave office or be forced out by UN-backed regional forces.
Troops have been told to halt their advance until the deadline passes.
The Economic Community of West African States (Ecowas) is acting in support of Adama Barrow, who was sworn in as the new Gambian president on Thursday.
His legitimacy as president, after winning last month's election, has been recognised internationally.
Last-ditch mediation talks, led by Guinea's President Alpha Conde, are due on Friday morning.
