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' - Republican Senator Susan Collins

"I don't know the basis for President Trump's assertion," Susan Collins,  Republican senator from Maine, 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.

FBI Director

FBI Director James Comey speaks at a conference at Boston College on March 8. Comey is expected to be asked about Trump's claims when he testifies at a rare public hearing on Monday. (Stephan Savoia/Associated Press)

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.

The Justice Department on Friday delivered documents to congressional committees to help clear up whether the Obama administration spied on Trump.

White House seeking probe into leaks to media

Republican Devin Nunes of California, who leads the House intelligence panel, said after receiving the material, he saw no evidence of wiretapping.

But Nunes, who served on Trump's transition team, joined the White House in seeking to shift attention away from the controversies by calling for investigations of leaks to the news media.

Nunes said on Fox News Sunday that leaks to reporters about former Trump national security adviser Michael Flynn were criminal and that his panel was probing whether other names were leaked.

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Devin Nunes talks to reporters on Capitol Hill on Wednesday. Nunes, who leads the House intelligence panel, said he saw no evidence of wiretapping. (J. Scott Applewhite/The Associated Press)

Trump has been dogged by allegations that his associates had ties to Russian officials. He fired Flynn last month after reports he had discussed sanctions with Russia's ambassador before Trump took office, without telling other White House officials.

"The one crime we know that's been committed is that one: the leaking of someone's name," Nunes said. "Were there any other names that were ... leaked out?"

Nunes also said ahead of Monday's hearing he had seen "no evidence" of collusion between Russia and Trump's team. But Schiff, the panel's top Democrat, said there was enough "circumstantial evidence" that he still had questions.

'President Obama is owed an apology'

Meanwhile, the White House has not backed down on Trump's surveillance claims.

The administration was forced to reassure key ally Britain after White House press secretary Sean Spicer repeated a Fox News analyst's claim that a British intelligence agency helped Obama wiretap Trump. The British government strongly denied it.

The issue led to an awkward moment on Friday at a joint news conference with visiting German Chancellor Angela Merkel when Trump was asked about the wiretap claims by a German reporter.

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Trump said he and German Chancellor Angela Merkel had 'something in common,' apparently referring to reports during the Obama administration that Merkel's phone was bugged. The quip left the German leader looking bewildered. (Saul Loeb/AFP/Getty Images)

Trump said he and Merkel had "something in common," apparently referring to reports during the Obama administration that Merkel's phone was bugged. The quip left the German leader looking bewildered.

Senior Republican Representative Tom Cole told reporters on Friday that Trump owed Obama an apology. Representatives Charlie Dent and Will Hurd, also Republicans, made similar comments.

"I see no indication that that's true," Cole said of the wiretapping charge.

Unless Trump produces convincing proof, Cole added, "President Obama is owed an apology."