In a letter to acting FBI Director Andrew McCabe, Mr Chaffetz set a deadline of May 24 for the FBI to produce "all memoranda, notes, summaries, and recordings referring or relating to any communications between Comey and the President."

Republican Speaker of the House Paul Ryan backed Mr Chaffetz.

"We need to have all the facts, and it is appropriate for the House Oversight Committee to request this memo," said Ryan spokeswoman AshLee Strong.

Comparisons with Nixon

Armed Services Committee Chairman John McCain said late on Tuesday that the developments had reached "a point where it's of Watergate size and scale."

"I think we've seen this movie before," the Arizona Republican said at a dinner to receive the International Republican Institute's Freedom Award. "The shoes continue to drop, and every couple days there's a new aspect."

Legal experts also compared the scandal to Watergate.

"For the president to tell the FBI to end a potential criminal investigation, that's obstruction of justice," said Erwin Chereminsky, a constitutional law professor and dean of University of California, Irvine School of Law. "This is what caused President Nixon to resign from office."

But the experts said intent was a critical element of an obstruction of justice charge, and the president's words could be subject to interpretation and possibly put into the context of other actions, like Mr Comey's termination.

Reports come in wake of Russia row

The latest bombshell came just as the White House was defending itself over claims Mr Trump revealed classified information to the Russians.

Officials said Mr Trump shared details about an Islamic State terror threat related to the use of laptop computers on aircraft with Russian Foreign Minister Sergey Lavrov and Ambassador to the US Sergey Kislyak.

The president hit back on Twitter, saying he had the "absolute right" to do so for "humanitarian reasons" and because he wants Russia to step up its fight against Isil.