Senegal troops have moved towards the Gambian border in an effort to force President Yahya Jammeh to accept electoral defeat and step down.
Senegal is leading the operation, which is supported by Nigeria and other states in the region.
Mr 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.
Mr Jammeh has ruled The Gambia since taking power in a coup in 1994.
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 Mr Barrow was preparing to be sworn in as president “on Gambian soil” on Thursday.
At least 26,000 Gambians, fearful that violence could erupt, sought refuge in Senegal this week.
Meanwhile, thousands of UK and Dutch tourists are being evacuated from the tiny West African state, which is popular with European holidaymakers because of its beaches.