Flynn has come under investigation for his ties both to Russian Federation and Turkey, and the Wall Street Journal reported Friday that the inquiry from Special Counsel Robert Mueller extends to a percolating plot against Poconos preacher Fetullah Gulen.
The meeting allegedly discussed a secret payout of up to US$15 million dollars (S$20.4 million) if, once in office, Mr Flynn would engineer the deportation to Turkey of Fethullah Gulen, a political rival to President Recep Tayyip Erdogan, as well as help free Iranian-Turkish businessman Reza Zarrab from prison.
He is believed to be under investigation for not registering as a foreign agent, a charge leveled at former Trump campaign chair Paul Manafort in an indictment from Mueller.
The people who described the alleged proposal said they didn't attend the December meeting and didn't have direct knowledge from Mr. Flynn or his associates about its purported details. Flynn has expressed interest in an immunity deal and Mueller, in his dealings with former foreign policy adviser George Papadopoulos, has shown himself ready to negotiate.
In September, the Wall Street Journal reported a meeting about the plan, in which former Central Intelligence Agency director James Woolsey participated. The discussions allegedly involved the possibility of transporting Mr. Gulen on a private jet to the Turkish prison island of Imrali, according to one of the people who has spoken to the Federal Bureau of Investigation.
Mr Flynn served 24 days as Mr Trump's national security adviser but was sacked after it was discovered he had misrepresented his contacts with a Russian diplomat to Vice President Michael Pence. It is not clear if any money changed hands or if any preparatory steps were taken.
While his investigation is focused on Russian meddling, Mr Mueller has a broad mandate and has homed in on Mr Flynn's activities as Mr Trump's lead national security adviser.
"If the facts involving Flynn and his son are true, or even mostly true, it indicates an incredible propensity for outrageously illegal conduct", said Ryan Goodman, a NY law professor and former Pentagon counsel.
Flynn's attorneys responded to the report Friday, denying the claims.
Former National Security Adviser Mike Flynn and the Turkish government are both refuting a report that they were considering a plan to forcibly send a Muslim cleric Turkey has said was behind a failed military coup from the United States to Turkey.