"I think it's totally ridiculous - everybody thinks so, and again we have to get back to working our country properly so that we can take care of the problems that we have", Trump said.
Asked if he urged Comey to drop the investigation of former national security adviser Michael Flynn - something a Comey associate claims happened based on extemporaneous notes kept by Comey at the time - Trump answered: "No. No".
Russian Federation has denied US intelligence agencies' conclusion that it interfered in the election campaign to try to tilt the vote in Trump's favour.
His appointment was met with approval from Democrats and Republicans on Capitol Hill, including Sen.
Senate Democratic Leader Chuck Schumer said Comey should still appear before congressional committees that have invited him to testify, despite Mueller's appointment.
"But if you're trying to build an obstruction-of-justice case, you might want the president to keep talking, because everything he does is digging a deeper legal hole for himself".
The Times reported that Comey wrote the memo immediately after meeting with Trump on February 14, one day after Flynn was asked to resign.
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".
In this August 21, 2013, file photo, then-FBI director Robert Mueller speaks during an interview at Federal Bureau of Investigation headquarters in Washington.
On the subject of the allegations - started in July 2016 by the Hillary Clinton campaign and embraced by Democrats and some of the media since her surprise defeat in November - that he and his campaign colluded with Russia, Trump said that "the entire thing has been a witch hunt".
"The White House stays out of criminal investigations, full stop".
The president said he thought the ousting would be greeted with bipartisan support.
He offered new justifications for his decision Thursday, even while referring to the Rosenstein memo as "a very, very strong recommendation".
Comey was abruptly fired by Trump at a time when he was leading an investigation into whether the latter's election campaign had links with Russian Federation during the United States presidential polls previous year. "And then on top of that, after the Wednesday performance by Director Comey, you had a person come and have to readjust the record, which many people have never seen before, because there were misstatements made".
During a speech at the Coast Guard Academy commencement Wednesday, an aggrieved Trump aired his mounting frustrations, declaring that "no politician in history, and I say this with great surety, has been treated worse or more unfairly".