US President Donald Trump will sign an executive order on social media companies on Thursday, White House officials said after Trump threatened to shut down websites he accused of stifling conservative voices.

The officials gave no further details. It was unclear how Trump could follow through on the threat of shutting down privately-owned companies including Twitter Inc.

The dispute erupted after Twitter on Tuesday for the first time tagged Trump's tweets about unsubstantiated claims of fraud in mail-in voting with a warning prompting readers to fact check the posts.

Separately, a three-judge panel of the US Court of Appeals in Washington on Wednesday upheld the dismissal of a lawsuit by a conservative group and...