Washington - US President Donald Trump has backed the US House Intelligence Committee chairman's efforts to investigate actions by US security and other officials under previous president Barack Obama, inserting himself into a political feud amid the panel's Russia probe.

"The big story is the 'unmasking and surveillance' of people that took place during the Obama Administration," Trump said in a tweet, one day after the committee's Republican chairman subpoenaed the CIA, FBI and National Security Agency.

Democrats shot back, accusing Trump, a Republican, of diverting attention from the ongoing scandal that now hangs over his young presidency and criticising committee Chairman Devin Nunes' subpoenas.

On Wednesday, Nunes asked the agencies for details of any requests made by two top Obama administration aides and the former Central Intelligence Agency director to "unmask" Trump campaign advisers inadvertently picked up in top-secret foreign communications intercepts.

In April, Nunes recused himself from leading the panel's investigation into suspected Russian meddling in the 2016 presidential election following a secret visit he paid to White House officials, but retains subpoena power.

Representative Jim Himes, a Democrat on the House Intelligence panel, replied to Trump's tweet, calling it a "pathetic distraction."

Ed Markey, a Democrat on the Senate Foreign Relations Committee, told CNN: "Nunes is too close to the Trump White House."

In a separate statement on Wednesday, Republican Representative Mike Conaway and Democratic Representative Adam Schiff, who are leading the committee's Russia probe, announced subpoenas for Trump's former national security adviser, Michael Flynn, and personal lawyer, Michael Cohen, as well as their firms.

Russian President Vladimir Putin has denied leading efforts to interfere in last year's US election, and on Thursday said some Russians might have acted on their own but not with their government's involvement.

Trump has denied any collusion between Russia and his campaign.

The House Intelligence Committee's investigation is one of several congressional probes into Russia, along with one by the Federal Bureau of Investigation.

Former FBI Director James Comey, whom Trump fired last month, could testify as early as next week and planned to confirm the president's pressure to drop the agency's investigation into Flynn, according to CNN.

Farage 'person of interest' in Trump-Russia investigation

Brexit campaigner Nigel Farage is a "person of interest" in the US investigation into possible collusion between Russia and Donald Trump's presidential campaign, the Guardian reported on Thursday citing unidentified sources.

The London-based newspaper said Farage had not been accused of wrongdoing and was not a suspect or target of the US investigation. But it said he was "right in the middle" of the relationships being looked at.

The newspaper said the former leader of the UK Independence Party had "raised the interest" of FBI investigators due to his connections with Trump and Julian Assange's Wikileaks, which published leaked emails from the Democratic National Committee during the campaign.

An FBI spokeswoman declined to comment on the Guardian story, and other US officials said they were unaware of any serious FBI interest in Farage.

Farage said on Twitter it had taken him a long time to read the Guardian article because he was "laughing so much at this fake news".

"This hysterical attempt to associate me with the (Vladimir) Putin regime is a result of the liberal elite being unable to accept Brexit and Trump," he said.

"I consider it extremely doubtful that I could be a person of interest to the FBI as I have no connections to Russia."

Accusations that Russia interfered in the US 2016 presidential election have dogged Trump since he entered office and a former FBI chief, Robert Mueller, has been named as special counsel to investigate any collusion.

US intelligence agencies concluded that Russia used computer hacking, propaganda and misleading news reports designed to sway political opinion in an attempt to boost Trump's chances of winning the White House.

Trump has dismissed the notion that Russia played any role in his November election victory. Kremlin chief Vladimir Putin, a former KGB spy, has denied meddling in the US election.

CIA Director Mike Pompeo called Assange's Wikileaks a "hostile intelligence service" in April after it distributed material hacked from Democratic National Committee computers during the 2016 campaign.

Pompeo said Russia's GRU military intelligence service had used Wikileaks to distribute the material and concluded that Russia stole the emails and took other actions to tilt the election in favor of Trump, a Republican, over Democratic candidate Hillary Clinton.

"One of the things the intelligence investigators have been looking at is points of contact and persons involved," the Guardian quoted one source as saying. "If you triangulate Russia, WikiLeaks, Assange and Trump associates the person who comes up with the most hits is Nigel Farage."

"He's right in the middle of these relationships. He turns up over and over again. There's a lot of attention being paid to him."

Farage, who campaigned for decades for Britain to leave the European Union, was a vocal backer of Trump, appearing on the campaign trail and meeting him in New York just days after the election victory.

Farage, who also attended Trump's inauguration in Washington, met Assange in March this year at the Ecuadorean embassy in London where the Wikileaks founder has been holed up for five years.