US President Donald Trump backed the House Intelligence Committee's efforts to investigate actions by US security and other officials under previous president Barack Obama, inserting himself into a political feud over its latest subpoenas in the Russia probe.

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

The big story is the "unmasking and surveillance" of people that took place during the Obama Administration. — Donald J. Trump (@realDonaldTrump) June 1, 2017

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

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JOSHUA ROBERTS/REUTERS Chairman of the House Intelligence Committee Devin Nunes.

On Wednesday (Thursday NZT), 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, according to congressional sources.

Another congressional source, who also requested anonymity, said Democrats were "informed and consulted" about the subpoenas ahead of time, but some committee aides said they were not.

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. A senior committee aide said Wednesday's subpoenas were not part of the Russia probe.

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."

Republican Representative Adam Kinzinger, a House Foreign Relations Committee member, also questioned Nunes' actions in a separate CNN interview.

Americans' names picked up in foreign communications intercepts must be concealed unless senior officials request them to be disclosed for intelligence or law enforcement purposes. Any such requests undergo rigorous legal reviews, and US officials have said all such requests under Obama, a Democrat, were conducted properly.

PUTIN DENIAL

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 (Friday NZT) 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 and has repeatedly questioned the US intelligence finding that Putin led an operation that included computer hacking, fake news and propaganda intended to swing the election in his favour over Democratic rival Hillary Clinton.

On Wednesday (Thursday NZT), Clinton said she suspected Trump's campaign guided Russian efforts.

The president's tweet also comes after White House spokesman Sean Spicer on Wednesday (Thursday NZT) directed any queries about the Russia investigations to Trump's outside counsel, Marc Kasowitz.

The House Intelligence Committee's investigation is one of several congressional probes into Russia, along with one by the Federal Bureau of Investigation. Also, the US Department of Justice recently appointed a special counsel.

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.

The Guardian newspaper on Thursday (Friday NZT) said leading Brexit campaigner Nigel Farage was a "person of interest" in the FBI probe but has not been accused of wrongdoing. Farage, a Trump supporter and former leader of the United Kingdom Independence Party, said he had no Russia connections.

Separately, the Washington Post reported that the Trump administration was moving toward returning two Russian compounds in the United States to Moscow.

The Maryland and New York compounds were seized under the Obama administration in December as part of a larger action over what the former president said was Russian involvement in hacking political groups in the November 8 US presidential election.

Russian officials had pledged to wait for Trump to take office before reacting but last month said they might retaliate.