The Republican and Democratic leaders of the US Senate Judiciary Committee asked Attorney General Jeff Sessions and FBI Director James Comey on Wednesday to send the committee documents and provide a briefing on the resignation of US President Donald Trump's national security adviser.

Citing reports that both the Federal Bureau of Investigation and Justice Department were involved in events leading to Michael Flynn's departure, senators Chuck Grassley and Dianne Feinstein said they raised "substantial questions" about Flynn's discussion with Russian officials.

Amid a deepening crisis over the relationship between Trump's aides and Russia, some senior Republicans on Wednesday issued their boldest challenge yet and vowed to get to the bottom of the matter, while Democrats demanded an independent probe.

Trump, facing rising unease among fellow Republicans in Congress less than a month into his presidency, sought to focus the attention on what he called criminal intelligence leaks about his ousted national security adviser, calling Flynn a "wonderful man" who was mistreated by the news media.

The New York Times reported on Tuesday that phone call records and intercepted calls showed members of Trump's presidential campaign and other Trump associates had repeated contacts with senior Russian intelligence officials in the year before the Nov 8 election in which Trump defeated Democrat Hillary Clinton.

Republican Trump critics including Senators John McCain and Lindsey Graham voiced fresh consternation, but comments by Senate Foreign Relations Committee Chairman Bob Corker, who has been a Trump supporter, increased the pressure on the White House.

Corker said the Russia issue was threatening Trump's agenda on foreign affairs and domestic matters like healthcare and tax policy. He questioned whether the White House was able to stabilize itself and said Flynn should testify before Congress.

Democrats, doubting that either Trump's Justice Department or the Republican-led Congress will pursue the matter vigorously, demanded an independent investigation

of possible illegal communications between Flynn and the Russian government and any efforts by Flynn or other White House officials to conceal wrongdoing.

The Democrats called for either a special counsel appointed by Trump's attorney general, Jeff Sessions, or the creation of a bipartisan commission with subpoena power. The top Senate Democrat, Chuck Schumer, said Sessions, a close ally of Trump, must recuse himself from any investigation.

But the top Republicans in the Senate and House of Representatives have insisted the matter be investigated by existing Republican-led committees.

US intelligence agencies previously concluded that Russia hacked and leaked Democratic emails during the election campaign as part of efforts to tilt the vote in Trump's favor.

Some experts expressed concern the White House could curtail or divert probes into Flynn and Russian involvement in the election unless Congress becomes more aggressive by holding hearings and appointing an independent commission or special prosecutor into whether Trump's team violated federal laws in their contacts with Russia.

Intelligence agencies now overseen by Trump may not be ideally suited to the job, they added.

"It's not, at the end of the day, the job of the intelligence community to regulate the White House - and it shouldn't be," said Stephen Vladeck, a University of Texas law professor who focuses on constitutional law and national security.

Flynn was abruptly forced out by Trump on Monday after disclosures he had discussed US sanctions on Russia with the Russian ambassador to the United States, before Trump took office, and that he had later misled Vice-President Mike Pence about the conversations.

The Defense Intelligence Agency, the Pentagon spy agency once headed by Flynn, formally suspended his security clearance allowing him access to classified information, DIA spokesman James Kudla said.

In Twitter posts on Wednesday, Trump called the reported Russian connection with his campaign team nonsense, adding: "The real scandal here is that classified information is illegally given out by 'intelligence' like candy. Very un-American!"

White House spokesman Sean Spicer said on Tuesday that Trump himself asked for Flynn's resignation.