Donald Trump speaks to the press during an announcement that Trump is investing in the development of luxury properties in the country of Georgia at a press conference in New York March 10, 2011. [Photo/Agencies]

They said the claims, which one called "still unsubstantiated," were contained in a two-page memo appended to a report on Russian interference in the 2016 election that US intelligence officials presented to Trump and President Barack Obama last week.

Trump responded on Tuesday evening in a tweet calling the reports: "FAKE NEWS - A TOTAL POLITICAL WITCH HUNT!" The Trump transition team did not immediately respond to a request for comment.

One of the officials, both of whom requested anonymity to discuss classified matters, said the Federal Bureau of Investigation and other US agencies are continuing to investigate the credibility and accuracy of the claims.

They are included in opposition research reports made available last year to Democrats and US officials by a former British intelligence official, most of whose past work US officials consider credible.

The official said investigators so far have been unable to confirm the material about Trump financial and personal entanglements with Russian businessmen and others whom US intelligence analysts have concluded are Russian intelligence officers or working on behalf of Russian intelligence. Some material in the reports produced by the former British intelligence officer has proved to be erroneous, the official said.

The FBI declined comment.