A top House Republican announced Wednesday (15 March) that there was no evidence to corroborate President Donald Trump's claim that former president Barack Obama ordered his campaign headquarters to be wire-tapped.

"We don't have any evidence that that took place," said Representative Devin Nunes (R-California) during a press conference on Capitol Hill. "I don't believe, just in the last week of time, the people we've talked to, I don't believe there was an actual tap of Trump Tower."

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The chief of the House Intelligence Committee also noted that the committee have asked the Justice Department, as well as the CIA, FBI and National Security Agency (NSA), which Trump aides may have been spied on through a loophole in US surveillance law, and then been exposed to the media via various leaks.

According to The Hill, Nunes specifically pointed to former national security advisor Michael Flynn, who was fired after it was revealed he had held conversations with Russian ambassador Sergey Kislyak. "We know for a fact that incidental collection on General Flynn was picked up," Nunes said.

"I remain concerned that there was additional incidental collection that we are not aware of," he added. "If additional names were unmasked, we're going to have to understand, were proper procedures followed and did official names get leaked to the media?"

The congressional committee will hold its first hearing on Monday (20 March), according to US News & World Report. Despite finding no evidence to support Trump's claim, Nunes promised to continue investigating.

Intelligence chiefs given tight deadline

Nunes and fellow committee head Representative Adam Schiff (D-California) gave FBI Director James Comey, CIA head Mike Pompeo and NSA head Michael Rogers until Friday to provide the information they requested, The Hill reported. The Republican lawmaker said the committee would "most likely" subpoena for information if it does not hear back by Friday.

The committee's letter to the agencies requested the names of any Americans whose identities were "disseminated in response to requests from [intelligence community] agencies, law enforcement, or senior Executive Branch officials ... that relate to Presidential candidates Donald J Trump and Hillary Rodham Clinton and their associates in 2016."

It also requested the names of agencies or executive branch officials that requested or authorised the disclosure and dissemination of those names.

Rogers and Comey are set to testify at Monday's hearing, while former CIA Director John Brennan, former Director of National Intelligence James Clapper and former Acting Attorney General Sally Yates are expected to testify at a hearing the following week.

Schiff also took aim at the president and White House Press Secretary Sean Spicer over the unsubstantiated allegations. "You can't level an accusation of that type without either retracting it or explaining just why it was done," he said.

"There are, from a national security perspective, great concerns if the president is willing to state things like that without any basis, because the country needs to be able to rely on him, particularly if we have a crisis that is an external crisis, as every president does within their term of office."