WASHINGTON • United States President Donald Trump cautioned fellow Republican Mitt Romney to be a "team player" after the former presidential candidate and incoming US Senator from Utah sharply criticised the President's actions and questioned his character.

In a Washington Post essay published late on Tuesday, Mr Romney suggested the US 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 Mr Romney, who takes up his new role in Washington today.

Mr Trump fired back in a tweet early yesterday morning that took aim at Mr Romney's failed bid for the White House in 2012.

"Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not. Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful. I won big, and he didn't. He should be happy for all Republicans. Be a TEAM player & WIN!" Mr Trump wrote on Twitter.

The President appeared to reference outgoing US Senator Jeff Flake, one of the few Republican lawmakers who publicly spoke out against Mr Trump and drew the President's wrath.

REUTERS