James Clapper. Thomson Reuters James Clapper, the former director of national intelligence, said Wednesday that President Donald Trump had asked him to "refute the infamous dossier" detailing explosive claims about his campaign's ties to Russia, but he "could not and would not" do so.

Clapper, who served under President Barack Obama from 2010 to 2017, described what he perceived as Trump's "internal assault on our institutions" in his disparagement of the US intelligence community and firing of James Comey as FBI director.

"The president-elect disparaged the intelligence community's high-confidence assessment of the magnitude and diversity of the Russian interference by characterizing us as Nazis," Clapper said at a National Press Club event in Australia.

Trump tweeted in early January that US intelligence agencies allowed the dossier, compiled by former British spy Christopher Steele and published in full by BuzzFeed, "to 'leak' into the public.'"

"One last shot at me," Trump wrote. "Are we living in Nazi Germany?"

Clapper said Wednesday that he "felt an obligation to defend the men and women of the United States intelligence community" after Trump's outburst. So he called Trump on January 11 and "tried, naively it turned out, to appeal to his higher instincts by pointing out that the intelligence community he was about to inherit is a national treasure," he said.

Trump, however, was interested in only one thing, Clapper said.

"Ever transactional, he simply asked me to publicly refute the infamous dossier, which I could not and would not do," he said on Wednesday.

Clapper released a statement on January 11 saying he had informed the president-elect that the US intelligence community had "not made any judgment that the information in this document is reliable." But Trump described the call differently, tweeting the next morning that Clapper had called him "to denounce the false and fictitious report that was illegally circulated."

Clapper, Comey, former CIA Director John Brennan, and NSA Director Mike Rogers briefed Trump and Obama on the existence of the dossier in early January, CNN reported at the time.

Some of the dossier's claims — many of which appear to align with events during the campaign — have been corroborated by the intelligence community. But many of its more explosive claims remain unverified.

CNN reported earlier in March that the FBI had information to suggest that the Trump campaign "communicated with suspected Russian operatives to possibly coordinate the release of information damaging to Hillary Clinton's campaign."

The congressional intelligence committees and the FBI are investigating as part of their probe into Russia's election interference whether anyone on Trump's campaign team colluded with Moscow during the election to undermine Clinton.

The FBI may be taking cues from the dossier because it has worked with its author in the past. Steele, who cultivated an extensive network of Russian sources during his time on MI6's Moscow desk, apparently worked with the FBI on matters related to Russia and Ukraine between 2013 and 2016 — specifically with the FBI's Eurasian Joint Organized Crime Squad, according to a lengthy profile in Vanity Fair.