Lashing out after a high-powered special counsel was appointed to investigate events around the 2016 election, Trump denied he ever plotted with the Kremlin to win the White House.
"The entire thing has been a witch hunt," Trump said during a press conference in the East Room of the White House.
"There is no collusion between certainly myself and my campaign, but I can always speak for myself, and the Russians -- zero."
Independent probe still needed, Pelosi says
On Wednesday, an investigation into his campaign's ties with Moscow was significantly strengthened by the appointment of straight-shooting former FBI director Robert Mueller to lead it.
Trump said he had "respect" for the move, but added: "I think it divides the country."
Early in the day, a seething Trump took to Twitter to blast the appointment, which shook Washington and sent world stock markets tumbling.
"This is the single greatest witch hunt of a politician in American history!" Trump wrote, his anger boiling over.
"With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special counsel appointed!" he said, without providing evidence for those claims.
US intelligence suspects that Russian President Vladimir Putin orchestrated a sweeping campaign to tilt the vote in the Republican's favor.
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Could President Trump be impeached?
At the center of the political firestorm are Trump's former national security advisor Michael Flynn and his one-time campaign manager Paul Manafort, and their multiple undisclosed contacts with Russian officials during and after the vote.
So far, no evidence of collusion has been presented, but the question has consumed the first four months of Trump's presidency and spurred multiple crises that have left the White House shell-shocked.
Crisis after crisis
Trump sent shockwaves across Washington last week by unceremoniously sacking his FBI director -- a virtually unprecedented move that came as James Comey was heading an investigation into Team Trump's links to Russia.
It later emerged that Comey had made notes of his meetings with Trump, who reportedly asked him to quash an investigation into Flynn's contacts with foreign governments.
Asked outright whether he had asked Comey -- as reported -- to drop the investigation into the Moscow connections of his sacked national security advisor, Michael Flynn, Trump fired back: "No. Next question."
"Director Comey was very unpopular with most people," he said. "We need a great director of the FBI."
Trump went on to dismiss as "totally ridiculous" the notion that he himself may have committed a prosecutable -- or even impeachable -- offense in recent months, as argued by some critics who suspect him of obstructing the FBI's probe.
"We look forward to getting this whole situation behind us," he said, vowing to focus his administration's resources instead on creating jobs, strengthening the military and reforming health care.
Special Counsel appointed to investigate Russian links to US election
At the start of another roller-coaster week in Washington, Trump was accused of divulging highly sensitive classified information gleaned by Israeli spies to visiting Russian Foreign Minister Sergei Lavrov.
The series of bombshell allegations has left Trump aides dazed and wondering about the future of his young presidency.
With Trump only eight percent of the way through its term, his advisors spend their days swapping rumors of mass White House firings.
Others are already considering leaving, and allies due to join the administration say they are holding off.
Ryan tries to tamp down Comey memo furore
The appointment of Mueller as special counsel will raise the intensity of the investigation and make it less vulnerable to political interference.
Deputy Attorney General Rod Rosenstein, who named Mueller to lead the probe, briefed US senators on Thursday.
Fighting back against multiple crises has consumed much of the political oxygen inside the White House, leaving Trump's agenda in doubt.
But the president's most powerful ally on Capitol Hill, House Speaker Paul Ryan, said Thursday the work of government would continue regardless.
"It's always nice to have less drama," he said. "That doesn't seize up Congress. That doesn't stop us from doing our jobs to work on people's problems."
Some of Trump's Republican allies, like Congressman Jason Amash, have begun cautiously suggesting impeachment is no longer impossible.
With the 2018 Congressional mid-term elections fast approaching, maybe of Trump's erstwhile allies face a difficult campaign dominated by questions about the president's conduct and with few legislative victories to fall back on.
A Gallup tracking poll put Trump's approval rating at a historically low 38 percent, even before the latest wave of scandals.
US administration governing 'appropriately': Bishop
Australia maintains a high level of confidence in its intelligence relationship with the United States despite recent events, the foreign minister says.
Pressure is mounting on President Donald Trump over interactions with Russia, including his reported disclosure of classified information with senior officials.
"The conversations the president has had are, to our understanding, within the type of conversations that one would expect leaders to hold," Julie Bishop told reporters in New York on Friday (AEST).
It follows meetings with several members of Trump's cabinet and administration, as well as the US Ambassador to the United Nations Nikki Haley.
Ms Bishop said the discussions were very positive and focused on the two countries' strong alliance.
They also spoke about President Trump's plan to cut the corporate tax rate and infrastructure agenda.
