Trump leaves domestic scandals behind during Saudi Arabia visit

US President Donald Trump left behind mounting scandals at home on Saturday, as he travelled to Riyadh to meet Saudi King Salman for his first overseas visit as president.

Descending the staircase of Air Force One in the Saudi capital, he showed little signs of being under pressure despite his team back in Washington coming under increasing scrutiny over alleged links to Moscow.

Shortly after the president's plane left the tarmac in Washington, former FBI chief James Comey - who was recently fired by Trump under mysterious circumstances - agreed to testify publicly about Russian interference in the US elections.

It came shortly after media reports that Trump told a delegation headed by Russian President Sergei Lavrov that Comey was "a nut job" and sacking the FBI director had relieved "great pressure" from him over an investigation into his alleged links with Russia.

"I just fired the head of the FBI. He was crazy, a real nut job," Trump said, according to the New York Times. "I faced great pressure because of Russia. That's taken off... I'm not under investigation."

Meanwhile, the FBI said they had identified a senior White House official as a "significant person of interest" in a probe into alleged Russian meddling in last year's US elections.

The Washington Post which disclosed the claims from sources, said it showed the investigation had reached the highest levels of government and that scrutiny is likely to intensify in the coming weeks and days.

The scandals have put Trump's approval ratings at the lowest level since he became president in January and brought huge questions over his ability to rule the country.

Investigators are looking into allegations that the Republican president's team worked with Russia to undermine Democrat challenger Hillary Clinton's campaign during last year's elections.

Clinton was cruising the polls until alleged emails of the former secretary of state were leaked, and became the focus of intense scrutiny by Trump and right-wing Americans.

Since then, allegations of collusion between Russia and the Trump team during this period have dogged the president.

Trump has declared himself the victim of the "greatest witch hunt" in American political history.

Before leaving for Saudi Arabia, the US president tweeted he would be "strongly protecting American interests" during his visit to Saudi Arabia, Israel and Europe.

He is set to unveil a bonanza military deal with Saudi Arabia in Riyadh.

Agencies contributed to this story.