Barack Obama's spokesman has denied allegations by Donald Trump that the former President ordered for his offices in New York to be “wiretapped” before the presidential election.
Speaking several hours after Mr Trump made the claims on Twitter, Kevin Lewis asserted that any suggestion Mr Obama or his staff had "ordered surveillance on any US citizen" was false.
He added that a “cardinal rule” of the Obama administration was that no White House official ever interfered in any Justice Department investigations, which are supposed to be conducted free of political influence.
“As part of that practice, neither Obama nor White House official ever ordered surveillance on any US citizen. Any suggestion otherwise is false,” Mr Lewis said.
The statement did not address the possibility that a wiretap of the Trump campaign could have been ordered by Justice Department officials.
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.
The President made the allegations against his predecessor early on Saturday morning, tweeting: “Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!”
Shortly afterwards, Mr Trump added: “Is it legal for a sitting President to be ‘wire tapping’ a race for president prior to an election? Turned down by court earlier. A new low!
"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 suggested that Mr Obama was "sick", writing: "How low has President Obama gone to tapp [sic] my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!"
It is not the first time Mr Trump has attacked his predecessor, having waged a years-long “birtherism” campaign alleging Mr Obama was not a US citizen and had a fake birth certificate.
The President was tweeting from his Florida seaside resort, Mar-a-Lago, where he was scheduled to meet with Attorney General Jeff Sessions and Homeland Security Secretary John Kelly before a dinner with officials also including adviser Steve Bannon and White House Counsel Don McGahn.
Several conservative news outlets and commentators have made allegations in recent days about Mr Trump being wiretapped during the campaign.
The President did not provide evidence that Mr Obama was responsible for surveillance on his property, and the White House did not immediately return requests for comment.
A former senior US official with direct knowledge of investigations by the Justice Department under the Obama administration denied there was any such investigation of Mr Trump or that his phones were tapped.
"This did not happen. It is false. Wrong," the former official said, adding that Mr Obama could not order this, and that it would have been taken to a judge by investigators, but investigators never did this.
The former deputy national security adviser under Mr Obama, Ben Rhodes, meanwhile tweeted that presidents cannot order wiretapping.
"No president can order a wiretap. Those restrictions were put in place to protect citizens from people like you," Mr Rhodes said in his Twitter post.
Members of Congress said the accusations require investigation or explanation.
Senator Ben Sasse, a Republican, described the allegations as serious and said the public deserved more information.
He said it was possible that Mr Trump had been illegally tapped, but if so, the President should explain what sort of invasion took place and how he knew about it.
Representative Adam Schiff, the most senior Democrat on the House Intelligence Committee, called Mr Trump's assertion a “spectacularly reckless allegation”.
“If there is something bad or sick going on, it is the willingness of the nation's chief executive to make the most outlandish and destructive claims without providing a scintilla of evidence to support them,” he said.
It was the latest twist in a controversy over ties between Mr Trump's associates and Russia that has dogged the early days of his presidency.
Mr Trump and the Kremlin refuted report by US intelligence agencies that concluded Russia hacked and leaked Democratic emails during the election campaign as part of an effort to damage Hillary Clinton.
The former national security adviser, Michael Flynn, resigned in February after revelations that he had discussed sanctions with the Russian ambassador to the US during the transition period.
Mr Flynn had promised Vice President Mike Pence he had not discussed the matter, but transcripts of intercepted communications showed that the subject had come up.
Attorney General Jeff Sessions was also forced to remove himself from any investigation into the administration's links with Russia after it was revealed he failed to declare his own meetings with the same ambassador.