A classified report presented to Donald Trump included allegations that Russian operatives claim to have compromising personal and financial information on him, US officials have confirmed.
The dossier reportedly claims that Russia had obtained "kompromat" (compromising material) on the President-elect during a stay in a five-star hotel in Moscow - including recordings of graphic sexual acts.
:: Russia: Donald Trump dossier claims 'fake'
It was also said to have included allegations of ongoing contact between associates of Mr Trump and intermediaries for the Kremlin.
Intelligence chiefs have reportedly been unable to verify the claims, which Russia has dismissed as "utter nonsense".
But the allegations were considered sufficiently serious to be included in a highly classified report on alleged Russian interference in the US presidential election.
US officials have confirmed Mr Trump and President Obama received the two-page memo - believed to be a synopsis of more detailed reports compiled by a British former spy.
The documents - apparently gleaned from sources in Russia and the US - claim the Russian regime has been "cultivating, supporting and assisting" Mr Trump for at least five years.
They suggest Vladimir Putin ordered his operatives to provide intelligence on Mr Trump's opponents, including Hillary Clinton, in return for information on US-based Russian oligarchs.
:: Did Russia spy on Trump when he was in Moscow?
And sources in the unverified memos claimed Russian authorities had obtained enough embarrassing evidence of Mr Trump's conduct while staying in the country to be able to blackmail him if necessary.
The material - reportedly compiled for Democratic and Republican opponents of the President-elect - has circulated for months in the US but has now been published in full by BuzzFeed.
Last October the former spy behind the memos told US website Mother Jones he judged the information "sufficiently serious" to share with the FBI.
A spokesman for Harry Reid has said the former Senate Democratic leader saw the memos before writing a public letter to FBI Director James Comey about Trump's ties to Russia.
In the letter, he wrote: "It has become clear that you possess explosive information about close ties and coordination between Donald Trump, his top advisors, and the Russian government - a foreign interest openly hostile to the United States."
According to CNN, Senator John McCain - chair of the Senate Armed Services Committee, which is investigating Russian hacking of the election - also gave a full copy of the memos to Mr Comey on 9 December.
As details of the dossier emerged, the President-elect tweeted: "FAKE NEWS - A TOTAL POLITICAL WITCH HUNT!"
Reince Priebus told NBC the "salacious garbage" in the report "didn't happen".
The Guardian reports that the FBI applied for a warrant from the foreign intelligence surveillance (Fisa) court over the summer in order to monitor four members of the Trump team suspected of irregular contacts with Russian officials.
The court turned down the request, asking FBI counter-intelligence investigators to narrow its focus.
Mr Comey refused to say whether the FBI was investigating allegations of links between Trump associates and the Kremlin when questioned by the Senate Intelligence Committee on Tuesday.
Mr Trump has consistently defied the consensus of 17 US intelligence agencies - playing down claims that Russia interfered with the election to help him win.
Last month he said: "It's time for our country to move on to bigger and better things."