WASHINGTON, May 27 (CGTN) - U.S. President Donald Trump threatened Wednesday to shutter social media platforms after Twitter for the first time acted against his false tweets, prompting the enraged Republican to double down on unsubstantiated claims and conspiracy theories.

Trump will sign an executive order "pertaining to social media" on Thursday, aides to the president said, without offering more detail about its contents.

Spokeswoman Kayleigh McEnany made the remark to reporters aboard Air Force One, traveling with Trump to Washington from Florida a day after Twitter Inc. attached a warning to some of his tweets prompting readers to fact-check the president's claims.

Trump on Wednesday threatened social media giants that his government could "strongly regulate" or "close them down" after Twitter fact-checked one of his tweets for the first time.

"Republicans feel that Social Media Platforms totally silence conservatives voices," Trump tweeted. "We will strongly regulate, or close them down, before we can ever allow this to happen."

The president's remarks came after Twitter slapped a warning label on one of his tweets on Tuesday, cautioning readers "Trump makes unsubstantiated claim that mail-in ballots will lead to voter fraud."

It was not immediately clear whether Trump has the authority to shut down the companies. Twitter declined comment on news of Trump's plans. Facebook and Google did not immediately comment.

The American Civil Liberties Union said the First Amendment of the U.S. Constitution limits any action Trump could take to regulate social media platforms.

Separately, a three-judge panel of the U.S. Court of Appeals in Washington on Wednesday upheld the dismissal of a suit brought by a conservative group and right-wing YouTube personality against Google, Facebook, Twitter and Apple alleging they conspired to suppress conservative political views.