A spokesman for the opposition coalition said the longtime leader risked transforming himself into a rebel leader.
Halifa Sallah, a spokesman for the opposition coalition that spurred president-elect Adama Barrow to victory over Jammeh in a December 1 poll, said the longtime leader risked transforming himself into a rebel leader if he tried to stay on beyond his mandate.
"Any president who loses constitutional legitimacy becomes a rebel," Sallah said.
"Anybody who is a military officer or civil servant who refuses to be under another constitutional authority obviously would also become a rebel," he added.
The Gambia's top brass have flip-flopped over whether they will remain loyal to Jammeh, drawing warnings from the international community.
On Saturday the regional ECOWAS bloc said west African leaders would attend Barrow's inauguration and would "take all necessary actions to enforce the results".
Sallah spoke after reading an address to the nation on Barrow's behalf that made clear the president-elect intended to take power in January once Jammeh's five-year mandate expired.
"The constitution orders that I assume office on the day the term of office of outgoing President Jammeh expires. He assumed office on 19th January 2012. His term expires in January 2017," Barrow said in the statement.
"On the day his term expires my term as the lawful President of The Gambia begins," he added.