United States General David Petraeus has resigned amid a scandal over his extramarital affair.

Washington - United States Attorney General Eric Holder on Thursday defended the FBI’s decision not to immediately brief Congress on a probe that led to the resignation of CIA chief David Petraeus.

The Justice Department and the FBI have been bitterly criticised by some lawmakers for keeping them in the dark about an inquiry into email harassment that ended up exposing that Petraeus had had an extramarital affair.

But Holder insisted that the inquiry had uncovered nothing that warranted briefing Congressional oversight committees.

“We felt very secure in the knowledge that a national security threat did not exist that warranted the sharing of that information,” he told reporters in New Orleans, in his first remarks addressing the scandal.

Holder said the case eventually “got to that point where we thought it was appropriate to share the information, (and) we did so”.

His remarks come nearly a week after the explosive scandal first came to light when Petraeus, a retired four-star general who was hailed for his handling of the conflict in Iraq, tendered his resignation.

Just three days after President Barack Obama's re-election, the fallen spy chief confessed to an extramarital affair with his 40-year-old biographer.

Lawmakers in Washington have been clamouring to know why they were not informed about the probe, which was launched months before the CIA chief stepped down, arguing that Petraeus's involvement made the case a national issue.

Petraeus is expected to testify on Capitol Hill on Friday about the deadly September 11 attack on the US mission in Libya. - Sapa-AFP