The sex scandal that led to the resignation of CIA director David Petraeus has widened, with the top US commander in Afghanistan under investigation for alleged “inappropriate communications” with a woman who is said to have received threatening emails from General Petraeus’ former lover.

President Barack Obama has delayed General John Allen's nomination as NATO's supreme commander pending an investigation into his email correspondence with a woman at the center of a sex scandal.

CIA director David Petraeus was shocked to learn last summer that his mistress was suspected of sending threatening emails warning another woman to stay away from him, former staff members and friends have told the Associated Press.

FBI agents are at the home of the woman whose affair with retired General David Petraeus led to his resignation as CIA director.

Members of the US Congress are demanding more details about the FBI investigation that unmasked an extramarital affair between ex-CIA director David Petraeus and his biographer.