Washington (CNN) Former acting FBI Director Andrew McCabe said Thursday that a recent whistleblower complaint about a communication between President Donald Trump and a foreign leader is "incredibly concerning."

"I think you are right. This President has a history of treating classified information and sensitive information questionably at best," McCabe, a CNN contributor, told CNN's John Berman on "New Day."

The concern was first revealed by The Washington Post, which reported that an official in the American intelligence community was so bothered by a "promise" Trump made to a foreign leader that the official filed a formal whistleblower complaint with the inspector general of the intelligence community, citing two former US officials familiar with the matter. CNN has not independently confirmed the detail about a "promise" made to the foreign leader.

The revelation has opened fresh concerns about Trump's dealings with foreign leaders, though the matter is shrouded in mystery as its details remain under wraps. McCabe, who also served as deputy director of the FBI, appeared baffled by the complaint Thursday, telling Berman that "it's unfortunate that we're left to kind of piece together ... the puzzle."

"But the fact that we now know it was the President and we know it involved some sort of a promise to a (foreign) leader is deeply concerning," McCabe said.

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