(CNN) President Donald Trump is ready to oust Lt. Gen. H.R. McMaster and find a new national security adviser before the North Korea meetings in May, multiple sources told CNN Thursday.

The move may be delayed because there's no final decision on a replacement, sources say. The timing of an announcement is unclear -- one source said it could come as soon as Friday, though others say that is unlikely.

White House press secretary Sarah Sanders pushed back on reports that McMaster may be headed out the door in a tweet, saying,"Just spoke to @POTUS and Gen. H.R. McMaster - contrary to reports they have a good working relationship and there are no changes at the NSC."

Any delay in the move is also because McMaster is trying to nail down his next steps, one of the sources said.

The shake-ups come as Trump signaled this week that he's prepared to dismiss aides with whom he's clashed as he works to surround himself with advisers more aligned with his populist agenda and freewheeling style.

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