US President Donald Trump on Saturday accused former President Barack Obama of wiretapping him in October during the late stages of the presidential election campaign, but offered no evidence to support the allegation.
"How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!" Mr Trump said in a series of tweets on his Twitter account early on Saturday.
"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!"
Mr Trump also called it "McCarthyism", in reference to a mid-20th century Senator who claimed the US government had been infiltrated with Communist spies.
Spokesman for Mr Obama Kevin Lewis said the accusation was "simply false".
"A cardinal rule of the Obama Administration was that no White House official ever interfered with any independent investigation led by the Department of Justice," he said in a statement.
"As part of that practice, neither President Obama nor any White House official ever ordered surveillance on any US citizen. Any suggestion otherwise is simply false."
The White House did not respond to a request to elaborate on Mr Trump's accusations.
Former Obama adviser Ben Rhodes strongly denied Mr Trump's allegations.
"No President can order a wiretap. Those restrictions were put in place to protect citizens from people like you," Rhodes wrote on Twitter.
In response to Mr Trump's tweet about a lawyer making a "great case", Rhodes responded: "No. They couldn't. Only a liar could do that."
Another former senior official told CNN it "did not happen. It is false. Wrong."
A spokeswoman for Mr Trump said the Republican President is "having meetings, making phone calls and hitting balls" at his golf course in West Palm Beach.
In one of the tweets, Mr Trump said the alleged wiretapping took place in his Trump Tower office and apartment building in New York, but there was "nothing found".
Mr Trump's administration has come under pressure from Federal Bureau of Investigation and congressional investigations into contacts between some members of his campaign team and Russian officials during his campaign.
House Democratic Leader Nancy Pelosi ridiculed Mr Trump's assertions.
"The Deflector-in-Chief is at it again. An investigation by an independent commission is the only answer," she wrote on Twitter Saturday.
Several Republicans on Saturday again urged an investigation into a series of intelligence-related leaks.
Obama imposed sanctions on Russia and ordered Russian diplomats to leave the United States in December over the country's involvement in hacking political parties in the November 8 US presidential election.
On Saturday, Representative Eric Swalwell, a Democrat who is a member of the House Intelligence Committee, told Fox News that Mr Trump "is not credible when it comes to talking about Russia".
Mr Swalwell downplayed Mr Trump's allegation.
"I think this is just the President up early doing his routine tweeting, he said. "Presidents don't wiretap anyone. These are pursued by the Department of Justice in accordance with the FBI and signed off by a judge."
Under US law, a federal court would have to have found probable cause that the target of the surveillance is an "agent of a foreign power" in order to approve a warrant authorising electronic surveillance of Trump Tower.
Several conservative news outlets and commentators have made similar allegations about Mr Trump being wiretapped during the campaign in recent days, without offering any evidence.
Mr Trump's national security adviser, Michael Flynn, resigned in February after revelations that he had discussed US sanctions on Russia with the Russian ambassador to the United States before Trump took office.
Mr Flynn had promised Vice President Mike Pence he had not discussed US sanctions with the Russians, but transcripts of intercepted communications, described by US officials, showed that the subject had come up in conversations between him and the Russian ambassador.
Mr Trump has often used his Twitter account to attack rivals and for years led a campaign alleging that Mr Obama was not born in the United States. He later retracted the allegation.
Reuters / Newshub.