ABOARD A US MILITARY AIRCRAFT - The top US commander in Afghanistan, General John Allen, is under investigation for allegedly inappropriate communication with a woman at the center of the scandal involving former CIA Director David Petraeus, a senior US defense official said on Tuesday, speaking on condition of anonymity.

Defense Secretary Leon Panetta said in a statement given to reporters flying with him to Australia that he asked that Allen's nomination to be Commander of US European Command and Supreme Allied Commander Europe be delayed and that President Barack Obama agreed.

Allen, who denies wrongdoing, will remain in his job, but Panetta encouraged the Senate to quickly act on approving his successor, General Joseph Dunford.

Petraeus resigned on Friday as director of the CIA, saying he had engaged in an extramarital affair.

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