Senegal has moved troops towards the Gambian border in an effort to force President Yahya Jammeh to accept electoral defeat and step down.
Col Abdou Ndiaye, a spokesman for the Senegalese military, said Ecowas had decided on the deadline to try to achieve a diplomatic solution.
"Things are getting into place and Ecowas forces are ready to intervene if needed after midnight if we can't find a diplomatic solution to the Gambian crisis," he said.
The Ecowas force is seeking UN Security Council endorsement to use "all necessary measures" to help remove Jammeh.
Jammeh has been told to leave office by the end of Wednesday and hand over to last month's election winner, Adama Barrow, who is currently in Senegal.
Wednesday was meant to be his last day in office but parliament granted him three more months in the post.
Garba Shehu, official spokesman for Nigerian President Muhammadu Buhari, said Barrow was preparing to be sworn in as president "on Gambian soil" on Thursday.
Source: QNA
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