Mr Jammeh initially accepted defeat in the 1 December poll, but later said it was flawed.
The Ecowas chairman said Senegal had been chosen to lead operations "to restore the people`s wishes" if needed.
President Jammeh has already said he will not be intimidated, saying Ecowas had no authority to interfere.
Mr Jammeh, who has ruled for 22 years, has lodged a case before the Supreme Court to annul the vote after the electoral commission changed some results.
The commission insists the outcome was not affected by an initial error and that property developer Adama Barrow won the poll and should be inaugurated on 19 January.
Gambia crisis: Senegal troops `on alert` if Jammeh stays on
23 December 2016
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Senegal`s troops are on alert to intervene in The Gambia if President Yahya Jammeh refuses to step down next month, the regional bloc Ecowas says.
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