Donald Trump, the US president-elect, 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."

On Sunday, Russian diplomats who were 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 has denied the allegations.

Mr Obama also closed two Russian-owned compounds and ordered sanctions against two Russian intelligence agencies.