Classified documents given to US President Barack Obama and president-elect Donald Trump last week included allegations that Russian operatives claim to have compromising personal and financial information about Mr Trump, according to US news channel CNN.
The channel said several US officials with direct knowledge of the briefings have told it that the allegations were presented in a two-page synopsis included in a report on Russian interference in the 2016 US election.
The allegations came, in part, from memos compiled by a former British intelligence operative, whose past work US intelligence officials consider credible. The FBI is investigating the credibility and accuracy of the allegations, which are based primarily on information from Russian sources, but has not confirmed many essential details in the memos about Mr Trump, according to CNN.
Trump's team last night said the reports have not been confirmed and the FBI have yet to confirm any of the claims.
Trump himself tweeted last night, writing online; "FAKE NEWS - A TOTAL POLITICAL WITCH HUNT!"
The classified briefings last week were presented by four of the most senior US intelligence chiefs - Director of National Intelligence James Clapper, FBI Director James Comey, CIA Director John Brennan, and NSA Director Admiral Mike Rogers.
One reason the nation's intelligence chiefs took the extraordinary step of including the synopsis in the briefing documents was to make Mr Trump aware that such allegations involving him are circulating among intelligence agencies, senior members of Congress and other government officials in Washington, a number of sources told CNN.
These senior intelligence officials also included the synopsis to demonstrate that Russia had compiled information potentially harmful to both political parties, but only released information damaging to Hillary Clinton and Democrats.
This synopsis was not an official part of the report from the intelligence community case about Russian hacks, but some officials said it augmented the evidence that Moscow intended to harm Clinton's candidacy and help Trump's, several officials with knowledge of the briefings told CNN.
The two-page synopsis also included allegations that there was a continuing exchange of information during the campaign between Trump surrogates and intermediaries for the Russian government, according to two national security officials.
Shortly after CNN's report, US website Buzzfeed posted what it claimed to be the full 35-page memo. Buzzfeed said it had not verified the report and that it contained some clear errors.
The documents claims secret contacts between Trump and Russia, as well as claiming Russia to have compromising information about Trump's personal life.
Sources told CNN that these same allegations about communications between the Trump campaign and the Russians, mentioned in classified briefings for congressional leaders last year, prompted then-Senate Democratic Leader Harry Reid to send a letter to FBI Director Comey in October, in which he wrote: "It has become clear that you possess explosive information about close ties and co-ordination between Donald Trump, his top advisers, and the Russian government - a foreign interest openly hostile to the United States."
CNN said it had confirmed that the synopsis was included in the documents that were presented to Mr Trump but couldn't confirm if it was discussed in his meeting with the intelligence chiefs.
Meanwhile, Mr Trump's choice for the next attorney general said yesterday America should not ban all Muslims from entering the US, as he faced a grilling in the Senate over his positions on issues of law and order.
Jeff Sessions, a staunchly conservative senator from Alabama, was rewarded for his support of Mr Trump's campaign with the high-profile role.
But his nomination caused consternation among those who accuse him of racism and of not supporting the rights of women and minorities. Mr Sessions was rejected for a position as a federal judge in 1986 after accusations of racism from fellow prosecutors and employees in his office.
As attorney general he will be responsible for enforcing laws relating to hate crimes and abortion - and telling Mr Trump he cannot do what he chooses, if it runs against the constitution.
Dianne Feinstein, the most senior Democrat on the judiciary committee, said it had received letters of concern from 400 civil rights groups, 1,400 law professors, a "broad spectrum of organisations against domestic violence", and 70 reproductive health groups.
Protesters dressed as the Ku Klux Klan heckled Mr Sessions during the hearing, raising their (70) voices to shout insults when he expressed support for keeping Guantanamo Bay detention camp open, and when he was defended by a senator against racism charges.
Hillary Clinton has paid an impassioned tribute to the power of diplomacy - containing a barely-hidden dig at Donald Trump - saying the United States will "weather every storm on the horizon" by staying true to its "universal values".