The US Attorney-General has accused President Trump of making his job "impossible" because of his constant tweeting.
In a highly unusual public attack, Bill Barr said: "I have a problem with some of the tweets. I cannot do my job here at the department with a constant background commentary that undercuts me."
"I think it's time to stop the tweeting about Department of Justice criminal cases," Barr told ABC News.
His interview came as Mr Trump stands accused of interfering with the sentencing recommendation for his former adviser, Roger Stone - prompting four Justice Department prosecutors to resign from the case this week.
The outburst was all the more remarkable as Mr Barr has emerged as a powerful defender of Mr Trump, earning the nickname of the "president's attorney" from critics.
Mr Barr has been at the centre of allegations that he decided - allegedly under pressure from Mr Trump - to overrule his own prosecutors and seek a lighter prison sentence for Mr Stone.
He has previously been criticised by Democrats and legal experts for seeming to assist the president during the independent investigation into whether Mr Trump was helped by a Russian campaign to influence the 2016 election.
The two men are so close that there was immediate speculation that Mr Barr's television interview may have been more about managing public opinion than a genuine outcry against Mr Trump's alleged interference in judicial affairs.
The controversy comes a week after the Senate acquitted Mr Trump of impeachment charges of abuse of power and obstruction of Congress related to pressure on Ukraine to announce investigations, including into Mr Trump's political opponent Joe Biden.
Another staunch ally of the president, Senate Majority Leader Mitch McConnell, supported Mr Barr's comments about the tweets.
"If the Attorney-General says it's getting in the way of doing his job, maybe the president should listen to the attorney general," McConnell told Fox News.
White House spokeswoman Stephanie Grisham said Mr Trump had "full faith" in Mr Barr.
"The president wasn't bothered by the comments at all," she said in a statement.
