A Nigerian military source told the BBC that the patrol vessel is currently sailing off the coast of Ghana after leaving from the commercial capital, Lagos.
The initial aim is to put on a show of force rather than to launch an attack.
Senegal is preparing ground troops ahead of Thursday’s deadline.
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| NNS UNITY |
The Gambia’s tiny army is no match for the regional powers. In recent years, President Jammeh has been promoting his loyalists, including army chief Ousman Badjie, to ranks beyond their competence.
This has further downgraded its military capabilities.
The regional body, Ecowas, has said that military intervention will be a last resort to bring an end to the political deadlock in The Gambia.
Nigeria's President Muhammadu Buhari has been leading mediation efforts to persuade Mr Jammeh to hand power to former estate agent Adama Barrow, who won the 1 December election.
Also,
Outgoing Gambian President Yahya Jammeh declares 90-day state of emergency two days before expected handover.
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