President Donald Trump turned to Congress on Sunday for help finding evidence to support his unsubstantiated claim that former president Barack Obama had Trump's telephones tapped during the election. Obama's intelligence chief said no such action was ever carried out, and a U.S. official said the FBI has asked the Justice Department to dispute the allegation.

Republican leaders of Congress appeared willing to honour the president's request, but the move has potential risks for the president, particularly if the House and Senate intelligence committees unearth damaging information about Trump, his aides or his associates.

Trump claimed in a series of tweets without evidence Saturday that his predecessor had tried to undermine him by tapping the telephones at Trump Tower, the New York skyscraper where Trump based his campaign and transition operations, and maintains a home.

Obama's director of national intelligence, James Clapper, said nothing matching Trump's claims had taken place.

"Absolutely, I can deny it," said Clapper, who left government when Trump took office in January. Other representatives for the former president also denied Trump's allegation.

FBI wants claim disputed

The FBI has asked the Justice Department to dispute Trump's allegations, a U.S. official told The Associated Press on Sunday. The official wasn't authorized to discuss the request by name and spoke on condition of anonymity.

No such statement has been issued by the Justice Department. DOJ spokeswoman Sarah Isgur Flores declined to comment Sunday, and an FBI spokesman also did not comment.

The New York Times reported that senior American officials say FBI Director James Comey has argued that the claim must be corrected by the Justice Department because it falsely insinuates that the FBI broke the law.

White House press secretary Sean Spicer said without elaborating Sunday that Trump's instruction to Congress was based on "very troubling" reports "concerning potentially politically motivated investigations immediately ahead of the 2016 election." Spicer did not respond to inquiries about the reports he cited in announcing the request.

(1/4) Reports concerning potentially politically motivated investigations immediately ahead of the 2016 election are very troubling. —@PressSec

(2/4) President Trump is requesting that as part of their investigation into Russian activity, the congressional intelligence committees —@PressSec

(3/4) exercise their oversight authority to determine whether executive branch investigative powers were abused in 2016. —@PressSec

(4/4) Neither the White House nor the President will comment further until such oversight is conducted. —@PressSec

Spicer said the White House wants the congressional committees to "exercise their oversight authority to determine whether executive branch investigative powers were abused in 2016."

'Tool of an authoritarian'

He said there would be no further comment until the investigations are completed, a statement House Democratic leader Nancy Pelosi took offence to and likened to autocratic behaviour.

"It's called a wrap-up smear. You make up something. Then you have the press write about it. And then you say, everybody is writing about this charge. It's a tool of an authoritarian," Pelosi said.

Spicer's chief deputy, Sarah Huckabee Sanders, said she thinks Trump is "going off of information that he's seen that has led him to believe that this is a very real potential."

Josh Earnest, who was Obama's press secretary, said presidents do not have authority to unilaterally order the wiretapping of American citizens, as Trump has alleged. FBI investigators and Justice Department officials must seek a federal judge's approval for such a step.

Earnest accused Trump of levelling the allegations to distract from the attention being given to campaign-season contacts by Trump aides with a Russian official, including campaign adviser Jeff Sessions before he resigned from the Senate to become attorney general. The FBI is investigating those contacts, as is Congress.

Intelligence committees inquiring

Senate intelligence committee chairman Republican Richard Burr, said in a statement that the panel "will follow the evidence where it leads, and we will continue to be guided by the intelligence and facts as we compile our findings."

Republican Devin Nunes, chairman of the House Intelligence Committee, said in a statement that the committee "will make inquiries into whether the government was conducting surveillance activities on any political party's campaign officials or surrogates."

The committee's top Democrat, Adam Schiff of California, said Trump was following "a deeply disturbing pattern of distraction, distortion and downright fabrication."

The office of House Speaker Paul Ryan referred questions to Nunes, while a spokesman for Senate Majority Leader Mitch McConnell, said McConnell would not tell the Senate committee how to do its work.

Trump said in the tweets that he had "just found out" about being wiretapped, though it was unclear whether he was referring to having found out through a briefing, a conversation or a media report. The president in the past has tweeted about unsubstantiated and provocative reports he reads on blogs or conservative websites.

How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy! —@realDonaldTrump

I'd bet a good lawyer could make a great case out of the fact that President Obama was tapping my phones in October, just prior to Election! —@realDonaldTrump

Just out: The same Russian Ambassador that met Jeff Sessions visited the Obama White House 22 times, and 4 times last year alone. —@realDonaldTrump

Is it legal for a sitting President to be "wire tapping" a race for president prior to an election? Turned down by court earlier. A NEW LOW! —@realDonaldTrump

The tweets stood out, given the gravity of the charge and the strikingly personal attack on the former president. Trump spoke as recently as last month about how much he likes Obama and how much they get along, despite their differences.

Obama spokesman Kevin Lewis said Saturday that a "cardinal rule" of the Obama administration was not to interfere in Justice Department investigations, which are supposed to be conducted free of outside or political influence.

Lewis said neither Obama nor any White House official had ever ordered surveillance on any U.S. citizen. "Any suggestion otherwise is simply false," Lewis said.