US President Donald Trump has backed a decision by his former national security adviser to seek immunity in congressional investigations of possible ties between his campaign and Russia, but there was no immediate sign that the request would be granted.

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 yesterday.

Testimony from Flynn could help shed light on the conversations he had last year with Sergei 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 investigations because they were upset about his November 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.

White House spokesman Sean Spicer told reporters he was not concerned that Flynn could provide information that could be harmful to the administration. He said Trump wanted Flynn to testify to "get this matter behind us" but declined to say specifically that he should be granted immunity.

"The president is very clear that he wants Mike Flynn to go and be completely open and transparent with the committee, and whatever it takes to do that he is supportive of," Spicer said.

Former national security adviser Michael Flynn. Photo: Reuters

TOO SOON

US Representative Adam Schiff, ranking Democrat on the House of Representatives intelligence committee, said it was too soon to consider immunity requests.

"As with any investigation - and particularly one that grows in severity and magnitude by the day - there is still much work and many more witnesses and documents to obtain before any immunity request from any witness can be considered," Schiff said in a statement.

Schiff said the panel would discuss any request with the Justice Department and the Senate Intelligence Committee, describing such a request "a grave and momentous step."

Senator Angus King, a member of the Senate Intelligence Committee, also said it was too soon to discuss immunity.

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 panel.

Officials told them the committee was not interested in any immunity discussions at that time.

The Wall Street Journal reported today 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 February 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 criticised 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."