Trump did not provide any evidence for the claims, which followed an interview on Fox News in which the allegations came up.
A senior U.S. official told NBC News that the allegations have no merit, and the official added that Trump did not consult with people within the U.S. government who would know the merits of the charge before sending out the claims on his favored communications platform.
“Terrible! Just found out that Obama had my ‘wires tapped’ in Trump Tower just before the victory. Nothing found. This is McCarthyism!” Trump wrote as part of a series of tweets Saturday morning.
An FBI spokeswoman said the bureau would have no comment.
NBC News reached out to former Obama administration officials and the White House for comment and clarification but had not heard back.
But Ben Rhodes, former deputy national security adviser to Obama, said in a tweet that presidents can’t simply order wiretaps as Trump suggests.
Ryan responded by saying: “I don’t think that’s the case.”
Trump, who is spending the weekend at his Mar-o-Lago resort in Florida, frequently tweets about topics that have appeared in conservative media outlets.
Trump’s campaign offices were located in Trump Tower in Manhattan. The Trump family occupies a multi-floor penthouse in the building which also hosts retail businesses, commercial office space and other private residences.
There have been reports that communications between Trump associates and Russians were picked up by intelligence agencies during the campaign as part of routine surveillance of the Russians. Trump and his aides have denied there were any improper contacts.
At a rowdy town hall in Clemson, South Carolina, on Saturday, Republican Sen. Lindsey Graham told his constituents that, if true, the allegations would be the “biggest political scandal since Watergate” and vowed to get to the bottom of Trump’s claims.
“I am very worried that our president is suggesting that the former president has done something illegally,” he said. “I would be very worried if, in fact, the Obama administration was able to obtain a warrant lawfully about Trump campaign activity with foreign governments.”
Trump’s allegations about Obama were part of a flurry of early-morning tweets that also defended Attorney General Jeff Sessions.
“The first meeting Jeff Sessions had with the Russian Amb was set up by the Obama Administration under education program for 100 Ambs……”
“The same Russian Ambassador that met Jeff Sessions visited the Obama White House 22 times, and 4 times last year alone.”
News of the meetings drew fire from critics who said Sessions had misled the Judiciary Committee in his January confirmation hearings.
Sessions recused himself from overseeing any Justice Department investigation into alleged Russian election interference earlier this week.
On Friday he said he would submit amended testimony and respond to senators’ questions over his contacts with Russia’s ambassador.
Sessions faces calls for his resignation over the issue but Trump has backed his man saying he did nothing wrong.
In recent days, Trump has sought to fight back over allegations about the communications between members of his team and Russian officials during and after the campaign.
He lashed out at Democrats Chuck Schumer and Nancy Pelosi Friday, tweeting a picture of Schumer meeting Russian President Vladimir Putin in 2003.
Trump said both Democrats should also be investigated for their ties to Russia.
Schumer responded on the social network: “Happily talk re: my contact w Mr. Putin & his associates, took place in ’03 in full view of press & public under oath. Would you & your team?”
On Feb. 13, National Security Adviser Michael Flynn resigned just after the Washington Post first reported that the Justice Department had informed the White House that Flynn could be subject to blackmail.
Trump acknowledged in January that Russia had a role in a series of cyberattacks on America but says it didn’t impact the election.