Jammeh, who has vowed to stay in power despite losing the December 1 election to rival Adama Barrow, promised to defend his country against any “outside aggression”.
The veteran leader initially conceded defeat in the vote, then changed his mind days later – raising fears that regional powers might have to intervene to oust him.
Last week, Marcel de Souza, commission president for ECOWAS, said the body had put standby forces on alert.
In his New Year speech broadcast on state TV, Jammeh decried “the resolution of ECOWAS on the current situation to implement the results of the election by whatever means possible”.
“It is in effect a declaration of war and an insult to our constitution,” he said.
“Let me make it very clear that we are ready to defend this country against any aggression.
“My government will never opt for such confrontation but defending our sovereignty is a sacred duty for all patriotic Gambians.”
Barrow’s surprise victory and Jammeh’s initial decision to concede after 22 years in power was initially seen as a moment of hope.
President Muhammadu Buhari has stepped in as an ECOWAS mediator to offer Jammeh an “honorable exit”, but Jammeh said the bloc could no longer fulfill that role.