US President Donald Trump has accused his predecessor of wire-tapping his phone a month before he was elected.
President Trump tweeted early on Saturday: "Terrible! Just found out that Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism!"
He went on to say that a court had earlier denied a wiretap request.
The US president has given no details to back up the claim - or suggested which court order he was referring to.
Media reports in the past few weeks have suggested the FBI had sought a warrant from the foreign intelligence surveillance court (Fisa) last summer in order to monitor members of the Trump team suspected of irregular contacts with Russian officials.
The warrant was first turned down but then approved in October, according to the media reports.
There has been no official confirmation and it is also not clear if this evolved into a full investigation.
There has been no comment yet from ex-President Barack Obama.
Mr Trump tweeted: "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 the Election."
He called the alleged tapping "a new low" and said "This is Nixon/Watergate" - referring to the most notorious political scandal of 1972, which led to the downfall of President Richard Nixon after a web of political spying, sabotage and bribery was exposed by the media.
He also called it McCarthyism - the persecution for US Communists and their allies led by Senator Joe McCarthy in the 1950s.
Mr Trump's tweets followed allegations made by conservative radio host Mark Levin, which were later picked up by Breitbart News, the website founded by Steve Bannon, now Mr Trump's chief strategist.
Mr Levin said there should be a congressional investigation into what he called Mr Obama's "police state" tactics in his last months in office to undermine Mr Trump's campaign.
Breitbart summarises Mr Levin's accusations, which say that "the Obama administration sought, and eventually obtained, authorisation to eavesdrop on the Trump campaign; continued monitoring the Trump team even when no evidence of wrongdoing was found; then relaxed the NSA (National Security Agency) rules to allow evidence to be shared widely within the government".
One of the first to react to Mr Trump's claims was Democrat Eric Swalwell, who sits on the House Select Committee on Intelligence. His Twitter feed simply carried an image of one of the president's tweets and said: "You're not credible."
Mr Trump has been reeling from accusations of links between his campaign team and Russia, following an intelligence report that Moscow was involved in hacking in order to get Mr Trump elected.
In the latest twist, his Attorney-General Jeff Sessions has been forced to remove himself from an investigation into the Russian role.
This followed revelations that he had met the Russian ambassador during the campaign, despite denying this at his confirmation hearings.
Mr Trump's National Security Adviser Michael Flynn was forced to resign after four weeks in office for misleading the White House over his contacts with the Russian envoy during the election campaign. Sanctions against Russia were allegedly discussed.
Referring to contacts with Ambassador Sergei Kislyak, Mr Trump tweeted: "Just out: The same Russian Ambassador that met Jeff Sessions visited the Obama White House 22 times, and 4 times last year alone."
Credit: BBC