Retired General Michael Flynn, who resigned only 24 days after becoming national security adviser, wants protection against "unfair prosecution" if he testifies before the intelligence committees of the US Senate and the House of Representatives, his lawyer, Robert Kelner, said on Thursday.
Testimony from Flynn could help shed light on the conversations he had last year with Sergey Kislyak, Russian ambassador to the United States, while national security adviser for Trump's presidential campaign.
Trump, a Republican, said in a tweet that Democrats were instigating the congressional probes because they were upset about his Nov. 8 victory over their party's candidate, Hillary Clinton.
"Mike Flynn should ask for immunity in that this is a witch hunt (excuse for big election loss), by media & Dems, of historic proportion!" Trump said.
Trump would not comment further when asked about Flynn during a White House meeting with US manufacturers.
Senator Angus King, a member of the Senate Intelligence Committee, said it was too soon to discuss immunity for Flynn.
A government official with direct knowledge of the case told Reuters that lawyers for Flynn raised the immunity request roughly 10 days ago with representatives of the Senate Intelligence Committee.
Officials told them the committee was not interested in any immunity discussions at that time.
The Wall Street Journal reported on Friday that Flynn had also sought immunity from the Federal Bureau of Investigation in exchange for testimony.
Flynn was forced to resign as national security adviser on Feb. 13 for failing to disclose talks with Kislyak about US sanctions on Moscow and misleading Vice President Mike Pence about the conversations, which occurred before Trump took office.
Congressional committees and the FBI are also looking into allegations of Russian meddling in the 2016 U.S. election campaign.
Russia has denied allegations that it hacked emails of Democratic groups and released information to tip the presidential election toward Trump.
In a CNN interview on Friday, King criticized Trump's remarks and defended the investigations.
"This is not a witch hunt," King said. "This is an effort to get to the truth of some very important questions.
"There is no doubt whatsoever that the Russians were behind an effort to interfere in our elections," he said. "To continue to deny that - it just flies in the face of all of the reality."
(Reporting by Susan Heavey, Doina Chiacu, Mark Hosenball and Steve Holland; Editing by Chizu Nomiyama and Lisa Von Ahn)
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