Jammeh seized power in a coup in 1994 when he was an army lieutenant and has ruled ever since
"ECOWAS has decided as a result of the refusal of President Yahya Jammeh to accept the verdict of the Gambian people in the elections of December 1, 2016, to deploy troops from its member states to The Gambia with immediate effect," a statement signed by the Communications Director at the presidency.
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"The objective is to create an enabling environment due to the effective enforcement of the rule of law, and, in accordance with the Constitution of The Gambia, facilitate the inauguration of the President-Elect, Adama Barrow, on Thursday, January 19, 2017," the statement added.
Meanwhile, Gambia's Parliament has adopted a resolution to allow President Yahya Jammeh to stay in power for three more months, starting from Wednesday, just hours before the end of his official term.
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The Gambia, following elections in 2016, 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.
He declared Gambia an Islamic Republic last year.