The ECOWAS commission president, Marcel de Souza, said this on Malian TV Thursday night.
Mr. Jammeh has vowed to stay in power despite losing a December 1 election to rival Adama Barrow, raising the possibility that regional powers may intervene to oust him if diplomacy does not succeed in persuading him to leave.
“We have put standby forces on alert if he does not (step down) on January 19 when his mandate ends,” De Souza said.
“No one has the right to oppose the will of the people.”
Barrow’s surprise victory and Jammeh’s initial decision to concede after 22 years in power was seen across Africa as a moment of hope.
But the president changed his mind a week later and said again on Tuesday that he would not step down, rebuffing efforts by West African leaders to persuade him.
Mr. Jammeh said the electoral count was flawed and that ECOWAS has no right to meddle in Gambia’s internal affairs.
The BBC quoted De Souza as saying in an interview that Senegalese troops would lead any military intervention. Senegal is Gambia’s only territorial neighbour and has a frequently stormy relationship with the country, having sent troops there during a 1981 coup.
Senegal has indicated that military action would be an absolute last resort.
Diplomats say ECOWAS would probably seek approval from the U.N. Security Council for the use of force. ECOWAS deployed troops, led by Nigeria, to Liberia and Sierra Leone during civil wars in the 1990s, setting a precedent for possible intervention.
A first step for raising pressure once the handover date passes is likely to be targeted sanctions by the U.N. and others, diplomats say, also raising the possibility that Mr. Jammeh could be offered asylum abroad.
Mr. Barrow’s supporters suggested on Thursday that the president might not immediately be prosecuted for alleged human rights abuses during his rule.
“Justice is absolutely essential … but we are going to take the route to truth and reconciliation,” said coalition spokesman Halifa Sallah at a meeting with the African Bar Association.
Gambia’s Supreme Court will hear a legal challenge on January 10 from Mr. Jammeh’s ruling party which wants to overturn the election result.