The US Senate intelligence committee has issued a subpoena demanding that former national security adviser Michael Flynn hand over documents relating to the panel's investigation into alleged Russian meddling in the 2016 elections.

The committee first requested the documents from Flynn in a letter on 28 April, but following the advice of counsel, the retired lieutenant general declined to comply.

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In a joint statement on Wednesday (11 May), Senators Richard Burr, the committee's Republican chairman, and Mark Warner, its top-ranking Democrat, announced the subpoena after Flynn failed to meet the original 9 May deadline.

Senator James Lankford, a Republican on the committee, told Fox News that "a large number of documents" have been requested.

"He didn't turn those documents over, so we have subpoenaed those documents," Lankford said. "We do expect them to be delivered to us."

Flynn is one of four former aides to President Donald Trump that the intelligence committee has requested documents from. Carter Page, Paul Manafort and Roger Stone are the others, but it is not clear at this stage if they have also received subpoenas.

Flynn, as the most senior of the four, became the focal point of concern about alleged Russian interference after it emerged he secretly discussed US sanctions with Sergey Kislyak, the Russian ambassador to Washington, in December.

He was then forced to resign from his role in February, when the conversations were revealed. However, President Trump said this was because Flynn had misled Vice President Mike Pence on the content of the discussions, and not due to any wider wrongdoing.

The subpoena was announced a day after Trump created shockwaves when he unexpectedly sacked FBI Director James Comey.

Comey was leading the bureau's investigation – a separate probe to that of the Senate intelligence committee – into alleged Russian collusion.

Both the Trump administration and Russia deny any involvement in the alleged activity.

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