WASHINGTON -- Punching back a day after his fired FBI director's damaging testimony, President Donald Trump on Friday 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.

Standing with the president of Romania, a NATO partner, Trump at last confirmed his commitment to the alliance's mutual defence pact, Article 5, uttering words he deliberately did not say when he spoke at NATO's gathering in Belgium last month. On Friday he said he was "committing the United States to Article 5."

He also accused Qatar, a key U.S. military partner, of funding terrorism "at a very high level," and said solving the problem in the tiny Persian Gulf nation could be "the beginning of the end of terrorism." It was a forceful endorsement of this week's move by Saudi Arabia, Egypt, Bahrain and the United Arab Emirates to cut off ties to Qatar, but a very different message from the one delivered just an hour before by Secretary of State Rex Tillerson. Tillerson had called on the Arab nations to immediately ease their blockade on Qatar.

Trump also saluted the United States' relationship with Romania and praised its contribution to the global fight against terror.

The president had previewed his attacks against Comey in an early-morning tweet that broke his previous day's silence on his favourite social media megaphone.

"Despite so many false statements and lies, total and complete vindication," Trump wrote. It was a stunning accusation, suggesting that the former FBI director had lied to Congress, while under oath.

He also seized on Comey's revelation that he had directed a friend to release contents of memos he'd written documenting his conversations with the president to a reporter.

"...and WOW, Comey is a leaker!" Trump wrote at 6:10 a.m. He derisively repeated the "leaker" moniker when speaking to reporters in the Rose Garden.

Trump's private attorney, Marc Kasowitz, seized on Comey's admission that he had orchestrated the public release of the information. Kasowitz is expected to file a complaint with the Justice Department inspector general next week, according to a person close to the legal team who agreed to speak before the filing on condition that the person's name is not used.

Richard Burr, the Republican chairman of the Senate intelligence committee, and Sen. Mark Warner, the top Democrat on the committee, both said Thursday they believed Comey's account of the events.

"And I think you saw today the overwhelming majority of the intel members, Democrats and Republicans, feel that Jim Comey is credible. Even folks who have been his critics don't question his integrity, his commitment to the rule of law and his intelligence," Warner said.

Associated Press writers Julie Pace, Mary Clare Jalonick, Josh Lederman and Deb Riechmann contributed to this report.