U.S. lawmakers from both parties said on Sunday they had seen no proof to support the claim by Republican President Donald Trump that his predecessor Barack Obama had wiretapped him last year, adding pressure on Trump to explain or back off his repeated assertion.

Several Republicans last week urged Trump to apologize for the allegations he made in a series of tweets on March 4. The maelstrom also caused tension with key U.S. allies and threatens to distract Republicans from campaign promises on health care and taxes.

"I don't know the basis for President Trump's assertion," U.S. Senator Susan Collins, a Republican, said on NBC's "Meet the Press." "I do believe he owes us that explanation."

Collins said she supported Trump as president, but she wouldn't side with him if he "misstated what the facts are."

FBI Director James Comey is expected to be asked about Trump's claims when he testifies at a rare public hearing on Monday about alleged Russian meddling in the 2016 presidential election. Russia has denied the assertion it was involved in hacked emails and other attempts to influence the race.

Representative Adam Schiff, the top Democrat on the House Intelligence Committee holding the hearing, called Trump's claims "patently false" and said he expected Comey to say as much on Monday.