Punching back a day after his fired FBI director's damaging testimony, President Donald Trump accused James Comey of lying to Congress and said he was "100 per cent" willing to testify under oath about their conversations.

Trump cryptically refused to say whether those private exchanges were taped - a matter at the heart of the conflicting accounts of what passed between them at a time when Comey was leading an FBI investigation into Russia's interference in the presidential election and its ties to the Trump campaign.

He asserted that nothing in Comey's testimony to the Senate pointed to collusion with Russia or obstruction of justice. "Yesterday showed no collusion, no obstruction," Trump said.

He further denied ever asking Comey for his "loyalty," contradicting Comey's detailed sworn testimony about a private dinner the two men had in the White House.

"No I didn't say that," Trump stated abruptly, taking questions at a joint press conference with Romanian President Klaus Iohannis in the Rose Garden. Asked if he would make that denial under oath, he said, "100 per cent."

Trump's aides have dodged questions about whether conversations relevant to the Russia investigation have been recorded, and so did the president, in series of teases.

"Well, I'll tell you about that maybe sometime in the very near future," Trump said. Pressed on the issue, he insisted he wasn't "hinting anything," before adding, "Oh you're going to be very disappointed when you hear the answer, don't worry."

The House intelligence committee sent a letter Friday asking White House counsel Don McGahn whether any tape recordings or memos of Comey's conversations with the president exist now or had existed in the past. The committee also sent a letter to Comey asking for any notes or memos in his possession about the discussions he had with Trump before being abruptly fired last month. The committee is seeking the materials by June 23.

Comey told the Senate intelligence committee Thursday about several one-on-one interactions with the president, during which he said Trump pressed him to show "loyalty," to back off on the FBI investigation of his former national security adviser, Michael Flynn, and to disclose that Trump himself was not under investigation.

Comey said he refused on all points, told senators of the detailed memos he had written after his conversations with Trump and said he hoped those conversations were taped because he is confident of their veracity.