WASHINGTON: US President Donald Trump on Thursday voiced rare criticism of two of his most steadfast aides, Secretary of State Mike Pompeo and Attorney General Bill Barr, less than a month before elections.

Fuming in a telephone interview with Fox Business, Trump, who has been forced off the campaign trail by a Covid-19 diagnosis, demanded that both cabinet members take action concerning the previous administration of Barack Obama.

Trump said Pompeo should find a way to release emails from Hillary Clinton, the 2016 Democratic presidential candidate, a longstanding cause for Republican activists who attack her use of a private server while secretary of state.

"They're in the State Department but Mike Pompeo has been unable to get them out, which is very sad, actually," Trump said.

"I'm not happy about him for that reason," Trump said. "He's running the State Department - you get them out."

Pompeo has until now been one of the few aides never to cross the mercurial Trump, who has ruthlessly criticized both Pompeo's and Barr's predecessors after firing them.

Ironically, Pompeo rose to political prominence for his fiery tone in questioning Clinton when he was a congressman looking into the deadly attack on the US mission in Benghazi, Libya.

Trump - who is trailing in polls to former vice president Joe Biden - has been urging Barr to prosecute members of the Obama administration for investigating his campaign's relationship with Russia.

"To be honest, Bill Barr is either going to go down as the greatest attorney general in the history of the country or he's going to go down as a very sad situation," Trump said.