Sources close to Michael Flynn have reportedly made clear that the former national security adviser will refuse to comply with a subpoena from the Senate intelligence committee seeking documents as part of its investigation into alleged collusion between the Trump campaign and Russia.

Attorneys for Flynn cited that the daily "escalating public frenzy against him," coupled with the appointment of a special counsel by the Justice Department, had created a dangerous legal environment for him to cooperate with the Senate.

The letter, obtained by The Associated Press news agency, goes on to lay out the case for Flynn's right to invoke the Fifth Amendment, a constitutional right that shields a witness from self-incrimination.

Any testimony provided by Flynn "could be used against him" given the current climate surrounding the Senate's probe into Russia's election meddling, the letter said.

Flynn was forced to resign from his post in US President Donald Trump's cabinet in February following reports that he had lied to top officials, including Vice President Mike Pence, about the nature of his contacts with Russian diplomats. Later evidence showed that he had also received payments from Russian and Turkish interests.

Flynn has sought immunity from "unfair prosecution" in exchange for his cooperation with the committee investigation, which he has so far not been granted.

Read more: Trump denies urging former FBI chief to drop Flynn inquiry

Legal experts have maintained that the former US Army lieutenant general was always unlikely to hand over the subpoenaed materials, as doing so would have effectively waived his right to constitutional protections.

Flynn is also being investigated as part of a separate FBI probe, in which Trump has been accused of obstruction of justice; first by allegedly urging then-FBI Director James Comey to drop the case against Flynn, before dismissing Comey altogether.

Roadblocks ahead

Flynn's refusal to cooperate could present a major barrier to both the FBI investigation headed by newly appointed special counsel Robert Mueller, and the separate Senate and House inquiries into Russia's election meddling.

Senate committee members remained resilient, however. Republican Senator James Lankford said Flynn was free to invoke his constitutional rights against self-incrimination. "We will get to the truth one way or another," Lankford tweeted. "We need facts, not speculation & anonymous sources."

Read more: Reports: Obama warned Trump about Flynn's Russian ties

Meanwhile, Democratic Senator Dianne Feinstein said Flynn's decision was "unfortunate but not unexpected," adding that the committee would gain information in other ways.

Trump has also defended his former adviser, despite having ousted him, advising him to strike an immunity deal in the face of what the president has dismissed as a "witch hunt."

Those comments offer a markedly different tone to remarks Trump made during the presidential campaign concerning his Democratic rival, Hillary Clinton.

Aides of the former secretary of state pleaded the Fifth Amendment in the probe over her private email server, prompting Trump to comment at a September 2016 campaign rally in Iowa: "You see, the mob takes the Fifth. If you're innocent, who are you taking the Fifth Amendment?"

dm/cmk (AP, Reuters)