Jammeh lost to opposition during the December 3 Gambian presidential elections but refused to step down, instead setting up a Supreme Court body to investigate "irregularities" that took place during voting.
However, ECOWAS president Marcel de Souza says the embattled leader would be booted out next month.
Speaking to journalists in Mali on Friday, Souza announced:
"The deadline is January 19 when the mandate of Jammeh ends.
"If he doesn’t go, we have a force that is already on alert, and this force will intervene to restore the will of the people.
"No one has the right to oppose the will of the people."