Washington (AFP) - US President Donald Trump on Sunday accused James Comey of cowardice by leaking accounts of his meetings with the president, days after the ex-FBI director testified that Trump sought to derail the Russian probe.

"I believe the James Comey leaks will be far more prevalent than anyone ever thought possible," Trump wrote in an early morning tweet. "Totally illegal? Very 'cowardly!'"

Sacked FBI chief Comey delivered his bombshell allegations at a Senate hearing Thursday, saying in his sworn testimony that he had asked a "friend" identified as a Columbia University law professor to release a memo of his conversations with the president to the press.

Comey said he had hoped releasing the information via the media would prompt the appointment of a special counsel to handle the Russia probe, a ploy that ultimately proved successful.

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He branded the president a liar and said Trump urged him to abandon the investigation into the former national security adviser Michael Flynn, an allegation Trump has denied.

On Friday, Trump's former campaign manager Corey Lewandowski criticized Comey as not "man enough" for having leaked the memo via his friend rather than doing it himself.

"He gave his notes to a Columbia law professor because he wasn't man enough to give the notes directly to the media when he wanted them out to the media," Lewandowski told NBC's morning show "Today."

Though Trump has lambasted Comey as a "leaker," he also claimed "total and complete vindication" following the ex-FBI chief's testimony, focusing on Comey's confirmation that Trump was not personally being probed.