WASHINGTON (BLOOMBERG) - US President Donald Trump denied trying to quash an FBI investigation of his former national security adviser and said there was no collusion by his campaign with the Russians who used hacking and disinformation to interfere with the 2016 presidential election.

"No, no - next question," Trump said when asked at a White House news conference on Thursday (May 18) whether he pressured then-FBI Director James Comey in February to drop an investigation into former White House adviser Michael Flynn's dealings with Russia and Turkey.

He said it is "totally ridiculous" to think he's done anything that would amount to an impeachable offense.

Trump called the multiple inquiries into Russian meddling in the US election a "witch hunt" that's unnecessarily dividing the country.

But said he respects the decision Wednesday (May 17) by his Justice Department to appoint former FBI Director Robert Mueller as a special counsel for a probe that has enveloped the White House in a political storm.

While Trump questioned the legitimacy of the investigation, it has thrown the administration off track and consumed its political capital. Many lawmakers from both parties welcomed the appointment of Mueller by Deputy Attorney General Rod Rosenstein as a way to restore confidence in government institutions and bring order to the probe.

Rosenstein was at the Capitol on Thursday afternoon to brief the entire Senate in a closed session about the status of the Russia investigation and about his role in the president's abrupt dismissal of Comey as FBI chief last week.

Trump said Thursday he thought firing Comey would be a popular decision because he'd been criticised by Democrats and Republicans alike for his handling of an investigation into Democratic presidential nominee Hillary Clinton's use of a private email server while she was secretary of state.

He said Comey turned in a "poor performance" in his most recent testimony to Congress and that Rosenstein had written a "very strong letter" outlining Comey's shortcomings.

Trump said he planed to announce his choice for a new FBI director "soon."

He said earlier in the day that former Connecticut Senator Joseph Lieberman, a Democrat turned independent, was one of the top contenders.

The selection is subject to Senate confirmation, and several leading senators from both parties have said they wanted the president to avoid picking anyone with a political background as the nation's top law enforcement official.

Congressional Probes While Mueller takes over the Justice Department investigation, probes in Congress were proceeding as well.

Republican Richard Burr of North Carolina, chairman of the Senate Intelligence Committee, and Mark Warner of Virginia, the ranking Democrat, said the panel will continue its own investigation and "engage with Director Mueller" on any potential conflicts.

The House Intelligence Committee requested documents from the Justice Department and FBI about the bureau's investigation and any records related to Comey's dismissal. Comey wrote detailed memos of his conversations with
Trump, including one in which the president allegedly asked him to drop a probe of Flynn.

Flynn was forced to resign in February after it was disclosed that he misled Vice President Mike Pence about the nature of his conversations with Russia's US ambassador. Flynn is under investigation for his contacts with Russia as well as his work on behalf of Turkey's government while he was a prominent campaign supporter of Trump.

He has become a central figure in the investigations in Congress.

Flynn Subpoena Burr, the Senate intelligence panel's chairman, said members are awaiting word from Flynn's attorneys whether he will comply with a subpoena for his records.

Rosenstein appointed Mueller in an order giving him broad authority to pursue the investigation into Russian meddling, including "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump."

"If the Special Counsel believes it is necessary and appropriate, the Special Counsel is authorised to prosecute federal crimes arising from the investigation of these matters," the order says.

That means he can issue subpoenas, present evidence to a grand jury and bring charges - all the powers of a federal prosecutor.

Markets have been roiled by the uproar in Washington on the risk that Trump's plans for tax cuts and infrastructure spending will get stalled while the controversies played out. The benchmark S&P 500 slumped 1.8 percent Wednesday, its worst day since Sept 9.

Stocks rebounded Thursday, with technology and bank shares resuming rallies as data reinforced optimism in the economy. The Mueller appointment will, at least temporarily, relieve some of the pressure on congressional Republicans who've publicly complained that the constant drama and serial controversies at the White House were hindering their ability to work on their policy agenda.

It also may give Trump's White House some relief from the barrage of questions from the media as Mueller takes the lead in an investigation that will mostly take place out of the public eye for now.

Another beneficiary may be Rosenstein, who was criticised for doing the White House's bidding after a memo he wrote criticising Comey's handling of the Clinton email probe last year emerged as a justification for the FBI chief's dismissal.

The deputy attorney general notified the White House of his decision to name a special counsel only after he signed the order designating Mueller for the job.

The White House was already reeling from the allegation that Trump asked Comey to drop the Flynn investigation, the revelation that the president may have inadvertently passed sensitive intelligence to two top Russian officials in the Oval Office and a suggestion he made that he might be secretly recording his conversations. All that was in the last seven days.

Trump will carry that baggage on his first overseas trip as president. He leaves Friday (May 19) for meetings in Saudi Arabia and Israel followed by summits with US Nato allies and with other leaders of the world's major economies. He also is scheduled to have an audience with the Pope.

Trump's political organisation has been sending out appeals for donations by portraying the president as under siege by the Washington establishment and the media. One sent by email on Wednesday was titled "Sabotage."

The campaign on Thursday said that it generated US$314,000 online, mostly through small donations.