Regional leaders warned last month that the 15-member ECOWAS would "take all necessary action to enforce the results" of the December 1 poll.
Jammeh retorted in a New Year's speech on Saturday night that the ECOWAS summit decision was "totally illegal" as it violated the principle of non-interference in the internal affairs of member states.
"It is in effect a declaration of war and an insult to our constitution. It is therefore absolutely unacceptable," said Jammeh.
"Let me make very clear that we are ready to defend the country against any aggression and there will be no compromise on this."
Opposition leader Adama Barrow was declared the winner of the ballot in the former British colony and Jammeh -- in power for 22 years -- initially conceded defeat.
But he reversed his decision a week later, condemning "unacceptable errors" by election authorities and calling for a new election.
The Gambia's top court said last month it was adjourning until January 10 a case filed by Jammeh to annul the vote results.
Jammeh's stance has stoked international concerns about the future of the small west African country, with the UN joining African leaders in calling for him to allow Barrow to take the oath of office as scheduled on January 19.
But the 51-year-old strongman has remained defiant.
"Given the unjustifiable and unprecedented anomalies... what we are simply and rightfully asking for is to return to the polls and allow the Gambians to elect who they want to be their president in free and fair elections," he said Saturday.ft