White House chief of staff John Kelly has grown increasingly frustrated with President Trump in recent months, even telling senior national security officials last month that he believed the President was becoming "unhinged," a senior administration official told CNN.

"He said the President was becoming unhinged," the official said, adding that Kelly appeared to say it in "a moment of frustration."

Kelly referred to the President as "unhinged" during a meeting with the Secretary of Defense James Mattis, Joint Chiefs Chairman Gen. Joseph Dunford, and senior National Security Council officials, the senior administration official said.

Kelly's comment came as the officials deliberated on the future of the US effort in Syria, following Trump's off-the-cuff comments saying he was determined to get US troops out of the country.

In another example of Kelly being unhappy, one official said Kelly used “salty language” about the President when Trump wasn’t thrilled with Homeland Security Secretary Kirstjen Nielsen.