Allen was Deputy Commander of Central Command, based in Tampa, prior to taking over in Afghanistan. He also is a veteran of the Iraq war.
In the meantime, Panetta said, Allen's nomination to be the next commander of U.S. European Command and the commander of NATO forces in Europe has been put on hold "until the relevant facts are determined." He had been expected to take that new post in early 2013, if confirmed by the Senate, as had been widely expected.
The military official said Allen is in Washington, where he was to testify at his confirmation hearing before the Senate Armed Services Committee on Thursday. Panetta said he asked committee leaders to delay that hearing.
The senior defense official said Panetta has not talked to Allen about the investigation, nor has he discussed the matter with President Barack Obama, although he consulted with unspecified White House officials before making the decision to seek a postponement of Allen's confirmation hearing.
Panetta did talk about the Allen matter with Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, who happens to also be in Perth for a meeting of American and Australian diplomatic and defense officials. Those talks were starting Tuesday with an official dinner.
With a cloud over Allen's head, it was unclear Tuesday whether he would return to Kabul, even though Panetta said Allen would remain in command. The second-ranking American general in Afghanistan is Army Lt. Gen. James Terry.
NATO officials had no comment about the delay in Allen's appointment.
"We have seen Secretary Panetta's statement," NATO spokeswoman Carmen Romero said in Brussels. "It is a U.S. investigation."
Panetta also said he wants the Senate Armed Services Committee to act promptly on Obama's nomination of Gen. Joseph Dunford to succeed Allen as commander in Afghanistan. That nomination was made several weeks ago. Dunford's hearing is also scheduled for Thursday.
Associated Press writer Slobodan Lekic in Kabul, Afghanistan, contributed to this report.
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