Media playback is unsupported on your device Media caption Not all Republicans are happy with the Comey firing

A US Senate panel investigating Russian interference in last year's election has issued a rare formal demand for documents from President Donald Trump's former national security adviser.

Michael Flynn has failed to voluntarily co-operate with the investigation, the Senate Intelligence Committee says.

He was forced to resign in February after failing to disclose the content of his talks with Russian diplomats.

Meanwhile, the fallout continues over the firing of the FBI director.

The White House maintained that James Comey was removed on Tuesday for his handling of the inquiry over Hillary Clinton's emails.

But senior Democrats said they believed he had recently asked the justice department for more resources for his Trump-Russia investigation.

Flynn: Documents demanded

The Senate Intelligence Committee said it issued a subpoena after Mr Flynn rejected its request on 28 April to submit documents relevant to the investigation.

Mr Flynn, a retired army lieutenant-general, misled the White House about discussing US sanctions against Russia with the country's envoy, Sergei Kislyak, before Donald Trump's inauguration in January.

Image copyright Reuters Image caption Mr Flynn was forced to resign in February

His links to Russia are being scrutinised by the FBI and the House and Senate Intelligence Committees, as part of wider investigations into claims Moscow sought to tip the election in favour of Mr Trump, and into contacts between Russia and members of the president's campaign team.

The rare use of a subpoena by senators makes it clear that the committee is forging ahead with its investigation into the alleged Trump-Russia links, the BBC's Laura Bicker in Washington says.

Flynn: No stranger to controversy

Timeline of events

Russia: The scandal Trump can't shake

Comey fired: Fallout continues

Reaction to Mr Comey's firing continued on Wednesday, with a White House spokeswoman saying that President Trump had been considering sacking the FBI director since he was elected.

But critics accuse the Republican president of firing the nation's top law enforcement official because he was leading the Russian inquiry.

Media playback is unsupported on your device Media caption Donald Trump was with Henry Kissinger when he told reporters James Comey was "not doing a good job" as FBI chief

The White House has rejected calls to appoint a special prosecutor to investigate allegations the Trump campaign colluded with the Kremlin over last year's election.

The Senate Intelligence Committee invited Mr Comey to testify next week.

In a farewell letter to staff, Mr Comey said he would not "spend time on the decision or the way it was executed".

"I have long believed that a President can fire an FBI Director for any reason, or for no reason at all," he wrote.

"It is very hard to leave a group of people who are committed only to doing the right thing," he added.

"My hope is that you will continue to live our values and the mission of protecting the American people and upholding the Constitution."

Watergate? Not quite.

Comey fired: Is this a cover-up?

Your questions answered

Media playback is unsupported on your device Media caption Trump's love-hate relationship with Comey over a tumultuous year

President Trump stood by his actions, saying Mr Comey was fired "because he was not doing a good job".

On Wednesday, Democratic senator Dianne Feinstein said she "understood" that Mr Comey had asked Deputy Attorney General Rod Rosenstein - who wrote the memo on which President Trump says the sacking decision was based - for more resources for the FBI investigation.

Another Democratic Senator, Richard Durbin, told US media be believed the reports to be true, although Justice department spokeswoman Sarah Isgur Flores rejected them as "totally false".

Media playback is unsupported on your device Media caption Democratic Senator Ben Cardin: 'We know that this involves Russia'

Either way, Republicans and Democrats vowed the House and Senate Intelligence Committees' investigations into the Russia claims would continue.

Republican Senator Lindsey Graham said if Mr Trump believed replacing Mr Comey would halt the inquiries "he made a big mistake".

At the centre of the storm - Deputy Attorney General Rod Rosenstein

Image copyright Reuters Image caption The White House says the decision to sack Mr Comey was based on a memo by Mr Rosenstein (above)

52-year-old Harvard graduate confirmed by US Senate as Deputy Attorney General on 25 April

Had strong bi-partisan backing with 94-6 vote in his favour

Overseeing federal investigation of alleged Russian interference in November's elections, after his boss, Attorney General Jeff Sessions, recused himself over meetings with Moscow's envoy in Washington

Appointed by President George W Bush as US attorney in Maryland and kept on by President Barack Obama

Reputation as apolitical and professional

Wrote memo detailing "serious mistakes" by Mr Comey, but did not expressly call for his removal

Threatened to resign after White House cast him as the prime mover in the firing, according to an anonymous source quoted by the Washington Post

Read more about Rod Rosenstein