The Pentagon says the top American commander in Afghanistan, Gen. John Allen, is is under investigation for alleged "inappropriate" e-mail communications, and a senior defense official says there is a "distinct possibility" the matter is connected with the FBI investigation that led to the resignation of former CIA director David Petraeus.

Defense Secretary Leon Panetta said in a written statement issued to reporters aboard his aircraft early Tuesday that the FBI referred the matter to the Pentagon on Sunday, and that he ordered a Pentagon investigation of Allen on Monday.

The defense official confirmed the investigation was due to allegations of inappropriate emails between Allen and Jill Kelley, a close friend of the Petraeus family who reportedly led the FBI to investigate communications between Petraeus and his biographer Paula Broadwell.

The FBI then discovered Petraeus and Broadwell had engaged in an extramarital affair, which led Petraeus to resign as director of the CIA on Nov. 9.