US Attorney General Bill Barr on Thursday delivered an extraordinary public rebuke of Donald Trump, saying the president's tweets were making his job at the Justice Department "impossible."

"I have a problem with some of the tweets," Barr said in an interview with ABC News, adding: "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's interview came as Trump stands accused of interfering with the sentencing recommendation for his former advisor, Roger Stone -- prompting four Justice Department prosecutors to resign from the case this week.

The outburst was all the more remarkable as Barr has emerged as a powerful defender of Trump, earning the nickname of the "president's attorney" from critics.

Barr has been in the eye of the storm over allegations that he decided -- allegedly under pressure from Trump -- to overrule his own prosecutors and seek a lighter prison sentence for Stone. 

A veteran Republican consultant, Stone was convicted in November last year of lying to Congress, tampering with a witness and obstructing the House investigation into whether the Trump campaign coordinated with Russia to cheat in the 2016 election.

Trump has denied that his tweets attacking the original stiff sentence of 87 to 108 months recommended for Stone -- and in support of Barr after it was reduced by more than half -- amounted to political interference.

When asked whether he had spoken with Trump about recommendations in the Stone case, Barr replied: "Never."

"I'm happy to say that in fact the president has never asked me to do anything in a criminal case."

Barr is due to testify next month to Congress where Democrats have voiced deep concerns about his conduct in legal matters involving the president.

In the interview, Barr said he had been "surprised" by the sentencing recommendation filed by prosecutors on Monday, and had intended to "amend and clarify" the department's position the following day -- when Trump fired off his tweet.

"Once the tweet occurred the question was, 'Now what do I do?' Do you go forward with what you think is the right decision or do you pull back because of the tweet -- and that just sort of illustrates how disruptive these tweets can be." 

Asked whether he was prepared for repercussions for speaking out against Trump, Barr replied: "Of course."

"I'm not going to be bullied or influenced by anybody," he said, "whether it's Congress, newspaper editorial boards, or the president, I'm going to do what I think is right."