The move marked an escalation in Trump's efforts to "investigate the investigators" as he undermines the findings of special counsel Robert Mueller's probe amid mounting Democratic calls for impeachment.

Press secretary Sarah Sanders said Trump is delegating to Barr the "full and complete authority" to declassify documents relating to the probe, which would ease his efforts to review sensitive intelligence.

Such a move could create fresh tensions within the FBI and other intelligence agencies, which have historically resisted such demands.

Barr has already asked John Durham, the US attorney in Connecticut, to examine the origins of the Russia investigation to determine whether intelligence and surveillance during the probe were lawful.

Trump has frequently claimed his campaign was the victim of "spying," though the intelligence community has insisted it acted lawfully in following leads in the Russia investigation and conducted surveillance under court order.

FBI Director Christopher Wray, though cooperating with Barr in the probe, would presumably baulk at declassifying information that could reveal sensitive sources or methods of FBI investigators.

Despite Mueller finding no evidence to support criminal charges against Americans related to Russia's actions, his report documented extensive Russian efforts to interfere in the 2016 campaign and willingness on the part of some in Trump's orbit to accept their aid.

After Mueller submitted his report to Barr in March, the Attorney General released a four-page summary to Congress. Barr's letter framed the debate about the probe over the next few weeks and, White House officials believe, allowed Trump to declare victory before the release of the full report, the contents of which are far more ambiguous.

Trump also appreciated Barr's combative stance with lawmakers and reporters as he defended the Justice Department's handling of the report.

Australian Associated Press