WASHINGTON — An American citizen who has been held in military custody in Iraq for four months has told lawyers with the American Civil Liberties Union that he wants to bring a lawsuit challenging his detention and wants the group to represent him, the organization said in a court filing on Friday.

The statements came from a man who, in September, surrendered to a Syrian militia, which then turned him over to American forces as a suspected Islamic State fighter. He made the statements in an unusual videoconference with A.C.L.U. lawyers on Wednesday, the court filing said. A federal judge last month had ordered the military to let the lawyers talk to the detainee.

The court filing was sparse on details, saying that during the Jan. 3 videoconference, the man informed the A.C.L.U. lawyers that “he wishes to continue this habeas corpus action” challenging the legality of his detention and asking to be released, and for the group to represent him.

The military has refused to identify the man other than to say that he existed and was being held as an enemy combatant. In October, the A.C.L.U. filed a lawsuit on his behalf challenging his detention. In an interview, Jonathan Hafetz, the lead A.C.L.U. lawyer on the case, said he was one of three attorneys who spoke to the man, and that they conversed in English.