Washington (CNN) On his first full day on the job, acting Secretary of Defense Patrick Shanahan told senior leaders at the Pentagon on Wednesday to "remember China, China, China," a US defense official told CNN.

"While we're focused on ongoing operations acting Secretary Shanahan told the team to remember China, China China," the official said.

Shanahan made the comments during a special meeting of all the military service secretaries and the undersecretaries of defense.

His decision to urge the Pentagon's leadership to focus on the threat posed by China came just hours before he attended a Cabinet meeting at the White House, where he sat next to President Donald Trump as Trump slammed Shanahan's predecessor James Mattis and the ongoing US military missions in Syria and Afghanistan.

"What's he done for me? How has he done in Afghanistan? Not too good. Not too good. I'm not happy with what he's done in Afghanistan and I shouldn't be happy," Trump said of Mattis, adding that he had "essentially" fired the retired general.

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