(CNN) The chairman of the Senate Armed Services Committee is urging President Donald Trump to quickly nominate a permanent secretary of defense.

Sen. James Inhofe, an Oklahoma Republican, warned on Tuesday that "we have too many threats facing this nation to have so many key DOD officials serving in an 'acting' capacity."

Acting Defense Secretary Patrick Shanahan, who took up the role in January following James Mattis' resignation, has been told by the Trump administration that his possible nomination for the permanent role is in a "holding pattern" after the Pentagon inspector general launched an investigation into possible ethics violations last week, according to a source close to Shanahan.

Inhofe told CNN on Tuesday that if the White House chooses "to nominate acting Secretary of Defense Shanahan," he would "welcome the DODIG's (Pentagon inspector general's) thorough, but expeditious examination of any outstanding issues."

A senior administration official said the White House isn't likely to move on nominating Shanahan while the investigation is underway. Administration officials tell CNN they are unsure how long the inspector general's investigation may take.

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