Donald Trump, the US president-elect, has claimed that "no computer is safe" from hacking, as he expressed continued scepticism that Russia interfered in the election.
Mr Trump said he knew "things that other people don't know" about alleged Russian cyber attacks, which he would reveal "on Tuesday or Wednesday".
"Hacking is a very hard thing to prove," he told reporters at his New Year's Eve party in his Mar-a-Lago private club in Florida. "So it could be somebody else. And I also know things that other people don't know, and so they cannot be sure of the situation."
Yesterday, Russian diplomats expelled by President Barack Obama left Washington.
"The plane has taken off, everyone is on board," said the Russian embassy, quoted by Russian news agencies.
The expulsions were part of a package of sanctions ordered by Mr Obama over Moscow's alleged interference in the US election. The Russian government denied all allegations.
Mr Obama also closed two Russian-owned compounds and ordered sanctions against two Russian intelligence agencies.
But Mr Trump questioned the reliability of US intelligence, pointing to failures over the existence of weapons of mass destruction in Iraq before the 2003 invasion.
"If you look at the weapons of mass destruction, that was a disaster, and they were wrong," he said. "So I want them to be sure."
Despite being a prolific user of Twitter, Mr Trump rarely uses computers or email. He advised people to avoid modern communications when dealing with important information.
"You know, if you have something really important, write it out and have it delivered by courier, the old-fashioned way," he said.
"Because I'll tell you what: no computer is safe."
Hundreds of guests attended the party at the Mar-a-Lago grand ballroom, including actor Sylvester Stallone and romance novel model Fabio.
Earlier in the day, Mr Trump left his press pool, travelling to play golf at one of his clubs and leaving journalists and the public with no knowledge of his whereabouts.
Yesterday, three former White House press secretaries criticised his treatment of the press.
Nicolle Wallace, communications director for former president George W Bush, said Mr Trump doesn't need the press, but wants it "like an addict craves their drugs".
Ari Fleischer, who also worked for Mr Bush, said there was a "double-barrelled hostility" between the president-elect and the press.
Joe Lockhart, the White House press secretary under president Bill Clinton, accused Mr Trump of creating his own facts. "It's somewhat Orwellian," he said in an interview with NBC. "You redefine the past, which means you can define the present and the future."
A member of Mr Trump's golf club in Jupiter, Florida, posted a photo on Twitter of Mr Trump on the greens on Saturday morning and said about 25 US secret service agents accompanied the president-elect. Reporters had not been advised of the visit to the club.
Transition aide Stephanie Grisham confirmed that Mr Trump had made a "last-minute trip" to Trump National Golf Club Jupiter, which is about a half-hour drive from Mar-a-Lago, where Mr Trump has been spending the holidays.
Ms Grisham said that she and other aides weren't aware of the trip and "appreciate everyone's understanding."
"We are in the home stretch of this transition period and don't anticipate any additional situations like this between now and inauguration," she said in a statement.
Before he went golfing on Saturday, Mr Trump tweeted an unusual New Year's message to friends and foes: "Happy New Year to all, including to my many enemies and those who have fought me and lost so badly they just don't know what to do. Love!"
With the arrival of 2017, another New Year's message moved on Mr Trump's Twitter account at about midnight.
This one was decidedly more upbeat, addressed to all Americans, and included an abbreviation for his campaign slogan: 'Make America Great Again'.
"To all Americans- HappyNewYear & many blessings to you all! Looking forward to a wonderful & prosperous 2017 as we work together to #MAGA." (© Daily Telegraph London)