The FBI is reported to be investigating allegations that Russian intelligence collected compromising information on Donald Trump.
The information reportedly compromises the president-elect personally and financially, and shows his aides were involved in organising the Russian hack of the Democratic Party during last year's election campaign.
US media have reported the information was included in an intelligence briefing received last week by both Mr Trump and President Barack Obama.
CNN described the briefing as a two-page synopsis "appended to a report on Russian interference in the 2016 election".
It said the allegations were based in part on memos from a former British intelligence operative, whose past work was considered credible by US intelligence officials.
Reuters quoted one US official, who requested anonymity to discuss classified matters, as saying investigators have so far been unable to confirm the material about Mr Trump.
Mr Trump has posted an apparent reference to the reports on Twitter, saying "fake news - a total political witch-hunt".
His spokesperson, Kellyanne Conway, moved quickly to dismiss them.
"Nothing's been confirmed. And I have to say, as an American citizen, regardless of your party or if you don't like politics at all ... we should be concerned that intelligence officials leaked to the press and won't go and tell the president-elect or the president of the United States himself, Mr Obama, what the information is."
None of the allegations have been verified yet.