WASHINGTON — The chiefs of the U.S. intelligence agencies presented President Barack Obama and president-elect Donald J. Trump with a summary of unsubstantiated reports last week that Russia had collected compromising and salacious personal information about Trump, two officials with knowledge of the briefing said.

The summary is based on memos generated by political operatives seeking to derail Trump’s candidacy. Details of the reports began circulating in the fall and were widely known among journalists and politicians in Washington.

The two-page summary, first reported by CNN, was presented as an appendix to the intelligence agencies’ report on Russian hacking efforts during the election, the officials said. The material was not corroborated, and the New York Times has not been able to confirm the claims. But intelligence agencies considered it so potentially explosive that they decided Obama, Trump and congressional leaders needed to be told about it and informed that the agencies were actively investigating it.

Intelligence officials were concerned that the information would leak before they informed Trump of its existence, said the officials, who spoke on the condition of anonymity because the summary is classified and talking about it would be a felony.

On Tuesday night, Trump responded on Twitter: “FAKE NEWS - A TOTAL POLITICAL WITCH HUNT!”

In an appearance recorded for NBC’s Late Night with Seth Meyers, Trump’s spokesperson, Kellyanne Conway, said of the claims in the opposition research memos: “He has said he is not aware of that.”

Since the intelligence agencies’ report on Friday that President Vladimir Putin of Russia had ordered the hacking and leaks of Democratic emails to hurt Hillary Clinton and help Trump, the president-elect and his aides have said that Democrats are trying to mar his election victory.

The decision of top intelligence officials to give the president, the president-elect and the so-called Gang of Eight — Republican and Democratic leaders of Congress and the intelligence committees — what they know to be unverified, defamatory material was extremely unusual.

The appendix summarized opposition research memos prepared mainly by a retired British intelligence operative for a Washington political and corporate research firm. The firm was paid for its work first by Trump’s Republican rivals and later by supporters of his Democratic opponent, Clinton. The Times has checked on a number of the details included in the memos but has been unable to substantiate them.

The memos suggest that for many years, the Putin government has looked for ways to influence Trump, who has travelled repeatedly to Moscow to investigate real estate deals or to oversee the Miss Universe competition, which he owned for several years. Trump never completed any major deals in Russia, though he discussed them for years.

The former British intelligence officer who gathered the material about Trump is considered a competent and reliable operative with extensive experience in Russia, U.S. officials said. But he passed on what he heard from Russian informants and others, and what they told him has not yet been vetted by U.S. intelligence.

The memos describe sex videos involving prostitutes with Trump in a 2013 visit to a Moscow hotel. The videos were supposedly prepared as “kompromat,” or compromising material, with the possible goal of blackmailing Trump in the future.

The memos also suggest that Russian officials proposed various lucrative deals, essentially as disguised bribes in order to win influence over the real estate magnate.

The memos describe several purported meetings during the 2016 presidential campaign between Trump representatives and Russian officials to discuss matters of mutual interest, including the Russian hacking of the Democratic National Committee and Clinton’s campaign chairman, John Podesta.

The first hint of the FBI investigation came in a Senate hearing on Tuesday in a series of questions from Senator Ron Wyden, the Oregon Democrat, to FBI director James Comey.

Wyden, trying to draw Comey out on information he may have heard during a classified briefing, asked whether the FBI had investigated the Trump campaign’s contacts with Russia. Comey demurred, saying he could not discuss any investigations that may or may not be underway. Wyden kept pressing, asking Comey to provide a written answer to the question before Trump’s inaugural on Jan. 20, because he feared there would be no declassification of the information once Trump took office.

After the hearing, Wyden tweeted: “Director Comey refused to answer my question about whether the FBI has investigated Trump campaign contacts with Russia.”

In fact, the material in the opposition research dossier had been given to the FBI before the election. But the FBI itself apparently struggled to confirm it, according to federal officials familiar with the investigation.

The FBI obtained the material long before the election, and some of the memos in the opposition research dossier are dated as early as this past June. But agents have struggled to confirm it, according to federal officials familiar with the investigation.

Allies of Senator Harry Reid, the Senate Democratic leader from Nevada who retired at the end of the year, said the disclosures validate Reid’s call last summer for an investigation by the FBI into Trump’s ties to Russia.

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“The evidence of a direct connection between the Russian government and Donald Trump’s presidential campaign continues to mount,” Reid wrote in a letter to Comey on Aug. 27.

Democrats on Tuesday night pressed for a thorough investigation of the claims in the memos. Representative Eric Swalwell of California, a member of the House intelligence committee, called for law enforcement to find out whether the Russian government had any contact with Mr. Trump or his campaign.

“The president-elect has spoken a number of times, including after being presented with this evidence, in flattering ways about Russia and its dictator,” Swalwell said. “Considering the evidence of Russia hacking our democracy to his benefit, the president-elect would do a service to his presidency and our country by releasing his personal and business income taxes, as well as information on any global financial holdings.”

With files from The New York Times

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