Former FBI director James Comey, who was fired by Donald Trump amid an agency probe into alleged Russian meddling in the US election, has agreed to testify before the Senate Intelligence Committee at a public hearing.

Key points: Mr Trump told Russian officials Comey was "crazy, a real nut job", The New York Times reported

Mr Trump told Russian officials Comey was "crazy, a real nut job", The New York Times reported White House did not deny the report, but said Mr Comey created "unnecessary pressure" on the Russia relationship

White House did not deny the report, but said Mr Comey created "unnecessary pressure" on the Russia relationship The news comes after former FBI director Robert Mueller was appointed special counsel to lead the Russia investigation

The hearing will be scheduled after the May 29 Memorial Day holiday, the committee said in a statement.

Mr Comey will certainly be asked about encounters that precipitated his firing, including a January dinner in which, Mr Comey has told associates, Mr Trump asked for his loyalty.

In the Oval Office weeks later, Mr Comey told associates, the President asked him to shut down an investigation into former national security adviser Michael Flynn.

Mr Comey is known to produce memos documenting especially sensitive or unsettling encounters, such as after the February meeting.

Earlier, the New York Times reported the US President had told Russian officials Mr Comey was a "nut job" whose ouster relieved "great pressure" on him.

The report cited notes from a May 10 Oval Office meeting — the day after Mr Trump fired Mr Comey.

"I just fired the head of the FBI. He was crazy, a real nut job," Mr Trump said, according to the document, which was read to The New York Times by a US official.

"I faced great pressure because of Russia. That's taken off."

In a statement in response, White House press secretary Sean Spicer did not deny the report, but said Mr Comey had created "unnecessary pressure" on the US' ability to conduct diplomacy with Russia.

"The President has always emphasised the importance of making deals with Russia as it relates to Syria, Ukraine, defeating ISIS and other key issues for the benefit and safety of the American people," the statement said.

"By grandstanding and politicising the investigation into Russia's actions, James Comey created unnecessary pressure on our ability to engage and negotiate with Russia.

"The investigation would have always continued, and obviously, the termination of Comey would not have ended it."

Separately, a Washington Post report, citing anonymous sources familiar with the matter, said a senior Trump adviser was now considered a "person of interest" in the law enforcement investigation into whether Mr Trump's campaign associates coordinated with Russia in an effort to sway the 2016 election.

The developments were a blow to White House efforts to tamp down interest in the Russia investigation as Mr Trump and his staff boarded Air Force One for Saudi Arabia — the first stop on his first foreign trip as President.

White House struggling to explain Comey's firing

The details of his comments to the Russians would seem to bolster theories Mr Trump fired Mr Comey in an effort to choke off the Russia investigation.

The White House has struggled to explain the chain of events that led to Mr Comey's firing and exactly who made the decision.

Mr Trump has insisted at times the decision was his alone, but he also has pointed to a "very strong" recommendation from Deputy Attorney-General Rod Rosenstein.

Mr Rosenstein told Congress he stands by a memo he wrote bluntly criticising Mr Comey.

But he made it clear it was not his intention for Mr Trump or other White House officials to use the document to justify firing Mr Comey, and he drafted it only after Mr Trump told him of his plans to dismiss the FBI director.

"My memorandum is not a statement of reasons to justify a for-cause termination," he said.

But he added: "I wrote it. I believe it. I stand by it."

Mr Rosenstein said, although he was personally fond of Mr Comey, "I thought it was appropriate to seek a new leader".

The news comes after it was last week announced former FBI director Robert Mueller had been appointed special counsel to investigate possible coordination between Russia and Mr Trump's campaign to influence the 2016 presidential election, in an effort to re-establish independence from the White House.

The appointment has drawn generally favourable comments from Democrats and from some Republicans as well.

House members and senators said Mr Rosenstein in his briefings steered clear of specifics in answering questions about his appointment of Mr Mueller, but made clear the former FBI director will have wide latitude to pursue the investigation — potentially including criminal charges.

Mr Trump has reacted furiously to the appointment, denying he had done anything that might merit prosecution or even impeachment, and then added of the lingering allegations and questions: "I think it's totally ridiculous. Everybody thinks so."

AP