In a barrage of tweets, United States president Donald Trump accused his predecessor, Barack Obama, of ordering a wire tap on him, but he offered no evidence for the claims.
White House spokesman Sean Spicer says there will be no further comment on the issue until Congress has completed its investigation.
That potentially would keep Mr Trump having to explain the allegations.
Mr Obama's former press secretary, Josh Earnest, says the former president never ordered the surveillance of any US citizen.
"This may come as some surprise to the current occupant of the Oval Office, but the president of the United States does not have the authority to unilaterally order the wire-tapping of an American citizen. If the FBI decided to use their wire-tapping authority in the context of a counterintelligence, or criminal, investigation, it would require FBI investigators, officials of the Department of Justice, going to a federal judge and making a case and demonstrating probable cause to use that authority to conduct the investigation. That is a fact."
The director of national intelligence at the time of the US election has also denied there was any wire-tapping of Donald Trump or his campaign.
James Clapper has told the US television network NBC he knew of no court order to allow monitoring of Trump Tower in New York.
"There was no such wiretap activity mounted against the President-Elect at the time, or as a candidate, or against his campaign."
The rival Democrats' Senate Minority Leader, Chuck Schumer, says President Trump is in trouble, whether or not the allegations are true.
"If he falsely spread this kind of misinformation, that is so wrong. It's beneath the dignity of the presidency. It is something that really hurts people's view of government. On the other hand, if it's true, it's even worse for the President, because that means that a federal judge, independently elected, has found probable cause that the President, or people on his staff, have had probable cause to have broken the law or to have interacted with a foreign agent."
Others suggest Mr Trump's outburst is an attempted diversion from questions over his administration's dealings with Russia.
House Minority Leader Nancy Pelosi has described Mr Trump as the "deflector in chief."
"This is ... it's called a 'wrap-up smear.' You make up something, then you have the press write about it, and then you say, 'Everybody's writing about this charge.' It's a tool of an authoritarian, to just have you always be talking about what you want them to be talking about. Rather than Russia, we're talking about President Obama. "
But the controversy has failed to deter supporters of Mr Trump, who gathered at a rally in Washington to back him.
Many directed their criticism at Barack Obama.
"If he did that and he kept listening to Trump when he wasn't supposed to, then they need to go after him for that, because that's illegal."
"Obama is a traitor. I believe he should be charged as such, and, if convicted, he should suffer the severest penalty."
"We're talking about our former president coming after our people, coming after our people and our vote. If he can do that to him, he'd have done that to any one of us."
