Berman's sudden resignation is likely to raise additional questions from congressional Democrats who have accused Barr of politicising the Justice Department and acting more like Trump's personal attorney rather than the nation's chief law enforcement officer.

The news comes just days after former national security adviser John Bolton claimed in his tell-all book that Trump promised Turkish President Recep Tayyip Erdogan he'd interfere in the Halkbank case that was being prosecuted in the Southern District.

The office has prosecuted a number of Trump associates, including Trump's former personal lawyer and fixer Michael Cohen, who served a prison sentence for lying to Congress and campaign finance crimes, and has also been investigating Giuliani and his associates.

Federal prosecutors in New York are investigating Giuliani's business dealings, including whether he failed to register as a foreign agent, according to people familiar with the probe. The people were not authorised to discuss the investigation publicly and spoke on the condition of anonymity.

Berman, a Republican who contributed to the president's election campaign, worked for the same law firm as Giuliani and was put in his job by the Trump administration. But as a US attorney, he won over some sceptics after he went after Trump allies.

He had recused himself from directly overseeing the Cohen investigation for reasons that were never disclosed.

Berman was appointed by then-Attorney General Jeff Sessions in January 2018, months after former US Attorney Preet Bharara was fired after refusing to resign along with dozens of other federal prosecutors appointed by President Barack Obama.

Three months later, FBI agents raided Cohen's offices, an act the president decried as a politically motivated witch hunt.

Berman was leading the Epstein inquiry following the death of paedophile Jeffrey Epstein, and has recently issued a statement saying that Prince Andrew, the Duke of York, is "falsely portraying himself to the public as eager and willing to co-operate".

Berman has taken a direct hand in other investigations that have angered Trump.

His office subpoenaed Trump's inaugural committee for a wide range of documents as part of an investigation into various potential crimes, including possible illegal contributions from foreigners to inaugural events.

And weeks before the 2018 midterm election, Berman announced insider trading charges against an ardent Trump supporter, Republican Republican Chris Collins. Collins, who represented western New York, has since resigned.

Under Berman's tenure, his office also brought charges against Michael Avenatti, the combative lawyer who gained fame by representing porn actress Stormy Daniels in lawsuits involving Trump. Avenatti was convicted in February of trying to extort sportswear giant Nike after prosecutors said he threatened to use his media access to hurt Nike's reputation and stock price unless the company paid him up to $25 million.

The Southern District of New York is one of the nation's premiere districts, trying major mob cases and terror cases over the years.

AP