US President Donald Trump has launched a Twitter tirade about the "guilt" of Hillary Clinton and the opposition Democratic Party, the BBC reported.

His Sunday morning outburst came amid reports that the first arrest in the Russian collusion inquiry would be made this week, possibly as early as Monday.

Trump insisted allegations of collusion between his campaign and Russia were "phony" and a "witch hunt".

He said Republicans were united behind him, before urging: "DO SOMETHING!"

Media reports say the first charges have been filed in the investigation led by special counsel Robert Mueller into alleged Russian interference in the 2016 election to assist Trump.