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. Sen. Susan Collins, a Republican, said on NBC's "Meet the Press." "I do believe he owes us that explanation."