President Donald Trump told Russian diplomats last week his firing of “nut job” James Comey had eased the pressure on him, as news broke that Mr Comey will testify at a public hearing on the Trump-Russia links.

Meanwhile, the FBI’s Trump-Russia investigation has moved into the White House, homing in on a senior Trump adviser, according to a Washington Post report.

The Post, citing anonymous sources familiar with the matter, said a senior Trump adviser is now considered a “person of interest” in the investigation into whether Mr Trump’s campaign associates collaborated with Russia in an effort to sway the 2016 election.

As Mr Trump left the United States on his first overseas trip, the bad news just kept coming, with the latest public opinion poll putting his approval rating at a dismal 38 per cent.

The New York Times reported that the President had told Russian officials he felt the dismissal of his FBI director had relieved “great pressure” on him.

The White House responded by saying Mr Comey’s firing was unrelated to the FBI’s Russia investigation.

The Senate intelligence committee announced that Mr Comey had agreed to testify at an open hearing at an undetermined date after Memorial Day.

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.

The new headlines were a fresh indication that Mr Trump would not be able to change the subject from what appears to be an intensifying investigation reaching toward the President and his inner circle.

The White House repeated its assertion that a “thorough investigation will confirm that there was no collusion between the campaign and any foreign entity”.

But it did not deny the Times report that Mr Trump was critical of Mr Comey to the Russians the day after he fired him.

The Times reported Mr Trump noted the Russia investigation as he told Russian Foreign Minister Sergey Lavrov and Ambassador to the US Sergey Kislyak of his decision to fire Mr Comey.

“I just fired the head of the FBI. He was crazy, a real nut job,” The Times reported that Trump said during the May 10 meeting.

I faced great pressure because of Russia. That’s taken off.”

White House spokesman Sean Spicer called the President’s rhetoric part of his deal-making.

“By grandstanding and politicising the investigation into Russia’s actions, James Comey created unnecessary pressure on our ability to engage and negotiate with Russia,” Mr Spicer said.

As for the separate report of a “person of interest” under investigation, the Post said the senior White House adviser “under scrutiny” is someone close to the President but did not name the person.

Earlier this week, the Justice Department appointed former FBI Director Robert Mueller to take over the federal investigation in an effort to re-establish independence from the White House.

The appointment of Mr Mueller as special counsel has drawn generally favourable comments from Democrats and from some Republicans as well. But Mr Trump has reacted furiously to the appointment.

The IPSOS poll charting Mr Trump’s declining popularity reflects the rising unease in Republican ranks. Among Republicans, 23 per cent expressed disapproval of Trump in the latest poll, up from 16 per cent in the same poll last week. The decline in support from Republicans appears to be a primary reason why Trump’s overall approval rating is now at the lowest level since he took office.

How Comey’s sacking went down

The details of Mr Trump’s comments to the Russians would seem to bolster theories Mr Trump fired Mr Comey in an effort to strangle the Russia investigation.

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”.