U.S. President Donald Trump on Wednesday cautioned fellow Republican Mitt Romney to be a "team player" after the former presidential candidate and incoming U.S. senator sharply criticized the president's actions and questioned his character.

In a Washington Post essay published late on Tuesday, Romney suggested the U.S. leader had "caused dismay around the world" and said his presidency had "made a deep descent in December."

"On balance, his conduct over the past two years ... is evidence that the president has not risen to the mantle of the office," said Romney, who starts his new role representing Utah on Thursday.