US President Barack Obama has put on hold General John Allen's nomination as Nato's supreme commander pending a probe into his email correspondence with a woman at the center of a sex scandal, a White House spokesman said on Tuesday.

"At the request of the secretary of defense, the president has put on hold his nomination of Gen Allen as SACEUR pending the investigation of Gen Allen's conduct by the Department of Defense IG (inspector general)," National Security Council spokesman Tommy Vietor said.

A US defense official said FBI investigators had uncovered a trove of 20,000 to 30,000 pages of correspondence -- mostly emails -- between Allen and Jill Kelley, a key figure in the scandal that brought down CIA chief David Petraeus.

"The allegations involve inappropriate communications" between Allen, the top US commander in Afghanistan, and Kelley, the official told reporters travelling with US Defense Secretary Leon Panetta.

Panetta said in a statement that his department was informed by the FBI on Sunday about the case and that he had referred it to the Pentagon's inspector general for investigation.

Allen, the top US commander in Afghanistan, had been tapped to take over as the Supreme Allied Commander Europe before the latest cascade of revelations.