US President Barack Obama assured citizens on Wednesday that the sex scandal that brought down CIA chief David Petraeus and ensnared another top general has not compromised national security.

Petraeus - the most celebrated US general of his generation, who was credited with turning around the war in Iraq - resigned last week to pre-empt revelations of an affair with his biographer Paula Broadwell, a married army reservist.

A lawyer acting for the US commander in Afghanistan, General John Allen, who is also under investigation over e-mails he sent to a Florida socialite, said his client would fully cooperate with the Pentagon probe.