US Attorney General Jeff Sessions

On Monday, Mr Trump had called his attorney general "beleaguered".

In the latest attack, the president said in a tweet on Tuesday that Mr Sessions had "taken a VERY weak position" on investigating his former opponent in the 2016 presidential election, Democratic nominee Hillary Clinton, over her use of a private email server.

CNN reported Wednesday that Russian ambassador Sergey Kislyak is considered by US intelligence to be one of Russia's top spies and spy-recruiters in Washington, citing current and former senior US government officials.

But asked by CNN's team in Moscow about the report, Russian Foreign Ministry spokeswoman Maria Zakharova insisted Kislyak was "a well-known, world-class diplomat."