WASHINGTON (AFP) - A US Senate panel issued a subpoena for documents from White House national security advisor Michael Flynn on Wednesday (May 11), taking the sprawling investigation into links between the Trump campaign and Russia to a new level.

The Senate Intelligence Committee said it made the formal demand after Flynn rejected its request two weeks ago for information on his contacts with Russian officials and businessmen.

The new order is for Flynn's documents "relevant to the Committee's investigation into Russian interference with the 2016 election."

Flynn, who was fired from his White House position after lying about December phone calls he had with Russian Ambassador Sergey Kislyak, is under investigation in Congress, the FBI and the Pentagon over his dealings with Russians dating back to late 2005.

In an email, his lawyer Robert Kelner said they had no comment at this time on the subpoena.

In March, Kelner requested immunity for Flynn to testify in Congress, saying the media was filled with "unfounded allegations, outrageous claims of treason, and vicious innuendo directed against him." "No reasonable person, who has the benefit of advice from counsel, would submit to questioning in such a highly politicised, witch-hunt environment without assurances against unfair prosecution," he said.

But so far the committees say they want to interview more witnesses and collect more documents.

A former US military intelligence chief, retired lieutenant general Flynn was generously paid to attend a gala of Russia's RT television in December 2015, where he sat with President Vladimir Putin, raising suspicions about his ties with Moscow.

His ouster on Feb 13 after less than four weeks as national security advisor came after he was shown to have lied to Vice President Mike Pence about his sensitive talks with Kislyak.

On Monday, the former acting attorney general, Sally Yates, told lawmakers that she had warned the president's top legal advisor on Jan 26 that Flynn's deception on those calls left him open to blackmail.

"We believed that General Flynn was compromised with respect to the Russians," she said.

"And that created a compromise situation, a situation where the national security advisor essentially could be blackmailed by the Russians."