The FBI is investigating possible links between US President Donald Trump's election team and the Russian government, Director James Comey told the House Intelligence Committee on Monday.

The intelligence agency is investigating Russian government interference in the US election and any links or cooperation between the Trump campaign team and Moscow, Comey told the panel, taking the unusual step of confirming an ongoing investigation.

The investigation is classified and Comey said he would not reveal more about the investigation or whom it is investigating.

Congressman Devin Nunes, a California Republican and chairman of the committee, said the panel aimed to get to the bottom of Russian actions and whether anyone from a political campaign conspired with them.

He also said the panel would look at allegations of "improper surveillance" after Trump charged that former president Barack Obama had order the wiretapping of his offices.

Nunes said lawmakers "know there was not a physical wiretap of Trump Tower," but said it was possible that other techniques were used against Trump and his associates.

Comey confirmed there was "no information" to support Trump's claims that the FBI wiretapped his offices.

"I have no information that supports those tweets and we have looked carefully inside the FBI," he said. "The Department of Justice has asked me to share that the information is the same for the department and all its components."

Comey said it was outside the power of the president to order a wiretapping of any US citizen.

In a more than five-hour hearing, Republicans on the committee focused on the damage caused by intelligence leaks, while Democrats focused on the Trump-Russia ties.

Nunes urged Comey to conduct a quick investigation because it was casting "a big, gray cloud that you've put over people who have very important work to do leading this country."

The White House however stood its ground, insisting there was no evidence Trump's team colluded with the Russians and that the president would still not withdraw allegations that Obama had ordered him to be surveilled.

"Investigating it and having proof of it are two different things," spokesman Sean Spicer said, noting the ongoing hearing in Congress was just the first of many.

Using his official government Twitter account, Trump had tweeted selective video clips from the hearing, claiming they showed "Russia did not influence the electoral process."

Comey however was asked about the reading during the hearing and declared it to be a misreading of his comments.

Trump had earlier weighed in before the hearing got underway to deny Russian meddling in the 2016 US elections.

"The Democrats made up and pushed the Russian story as an excuse for running a terrible campaign," Trump wrote on Twitter.

US intelligence agencies last year declared that hacking against Democratic Party organizations, which led to leaks of embarrassing information about the left-leaning party and its presidential nominee, Hillary Clinton, was likely directed by the highest levels of the Russian government.

Admiral Mike Rogers, head of the National Security Agency, told the panel that the intelligence community stands by that assessment.

The furore about the claims has mushroomed in recent months, with allegations that Russian officials held sway over Trump's campaign dogging his young administration for weeks now.

Those charges have proven hard to shake, given that Trump's first national security advisor, Michael Flynn, resigned after it emerged that he lied about contact with Russian officials before entering office.

Similarly, US Attorney General Jeff Sessions has been accused of lying about meetings with Russian officials.

But Trump, in his tweets, stood firm, noting that former director of national intelligence James Clapper said during a televised interview Sunday that he was not aware of any evidence linking Trump or his team to Russia.

"James Clapper and others stated that there is no evidence Potus colluded with Russia. This story is FAKE NEWS and everyone knows it!" tweeted Trump.

The president argued instead that the true problem is that so much information about the inner workings of his administration are being leaked to the media.

"The real story that Congress, the FBI and all others should be looking into is the leaking of classified information. Must find leaker now!" he tweeted.

As president, Trump has the authority to direct the FBI to launch such a probe.

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