A US Senate panel investigating Russian interference in last year's election has issued a subpoena for former national security adviser Michael Flynn.

The Senate intelligence committee said the subpoena was for documents relevant to its inquiry into Russian interference in the 2016 US presidential election, which Flynn declined to provide when the committee first requested them last month.

In a joint statement, Senators Richard Burr, the committee's Republican chairman, and Mark Warner, its top Democrat, said the committee had first requested the documents from Flynn in a 28 April letter, but the retired lieutenant general had declined, through counsel, to cooperate with the committee's request.

Flynn was forced to resign in February as Trump's national security adviser for failing to disclose the content of his talks with Sergey Kislyak, Russia's ambassador to the United States, and then misleading Vice President Mike Pence about the conversations. He has been a focus of investigations into Russia and the election

The subpoena was announced a day after Trump abruptly fired FBI Director James Comey, who had been leading the bureau's investigation of Russia and the election. Comey's firing prompted a storm of criticism from Democrats, who accused the president of seeking to stall the probe.