The FBI has asked the US justice department to refute President Donald Trump's accusation that Barack Obama ordered a wiretap of his phone, according to US media.
Citing senior officials, the New York Times reported that FBI director James Comey made the unprecedented request on Saturday because there was no evidence, and the accusation "insinuates that the FBI broke the law".
A US congressional committee has confirmed that it will look into Trump's allegation. The justice department has not commented.
Comey's request came after Trump, without providing evidence, accused Obama of ordering his phone in Trump Tower be tapped during last year's election campaign.
Trump has not commented further on the accusation since making it in a series of tweets on Saturday.
Democrats accused Trump of trying to distract from a rising controversy about possible ties to Russia. His administration has come under pressure from the FBI and congressional investigations into contacts between members of his campaign team and Russian officials.
A spokesman for Obama denied the allegation, calling it "simply false". The former president's national intelligence chief James Clapper has also denied it.
"There was no such wiretap activity mounted against the president-elect at the time, as a candidate at the time or against his campaign," Clapper said on NBC's Meet the Press.
White House backpedals
Jason Johnson, a professor of political science in Baltimore, told Al Jazeera that any investigation was likely to be short.
"It's not clear what the congressional committee will do and how they will do it, but it can't last very long because the accusation is false," Johnson said, adding a president is unable to order a wiretap.
"The president can't demand that anyone gets wiretapped and the only way that there could be a wiretap of Trump Tower is if judges and lawyers and investigators and the justice department felt that Trump was engaging in some illegal behaviour."
On Sunday, the White House appeared to somewhat backpedal on the accusations, only going as far as saying there should be a probe into the possibility of wiretapping.
WATCH: Is the media 'normalising' Donald Trump? (24:59)
"I think the bigger thing is let's find out. Let's have an investigation. If they're going to investigate Russia ties, let's include this as part of it," Trump spokeswoman Sarah Sanders said.
Trump's accusation appears to have been based on unverified reports from right-wing Breitbart News outlet, which was once run by his chief strategist, Steve Bannon.
Trump was said to be furious that positive reviews of his joint address to Congress on Tuesday were overshadowed by a series of revelations about meetings between members of his team and Russian officials.
The president was also said to be angry that Attorney General Jeff Sessions recused, or excused, himself from any election campaign or Russia-related investigations.
Democrats and a growing number of Republicans in Congress have called for the appointment of a special prosecutor and a bipartisan inquiry to look into the Russia question.
Source: Al Jazeera and news agencies