A key U.S. Republican congressman said on Wednesday he has seen no evidence that the Obama administration wiretapped Trump Tower during the 2016 presidential campaign, adding pressure to FBI Director James Comey to provide evidence supporting or debunking Republican President Donald Trump's claim.

"We don't have any evidence that that took place," said House of Representatives intelligence committee chair Devin Nunes. "I don't think there was an actual tap of Trump Tower."

Trump made the claim on Twitter on March 4 without providing evidence, and representatives of former Democratic president Barack Obama have denied it. A number of congressional committees added the startling accusation to their wider investigations of possible Russian meddling in the election campaign and Russian ties to Trump and his associates.

The top Democrat on the House intelligence committee, Adam Schiff, said Comey would be asked about wiretap evidence at a rare public hearing on Monday. "It deeply concerns me that the president would make such an accusation without basis," he said.

APTOPIX Trump Russia

Devin Nunes, the chairman of the House Intelligence Committee Chairman, right, and committee member Adam Schiff said they have no evidence to back up President Trump's claim that former President Barack Obama wiretapped Trump Plaza. (J. Scott Applewhite/The Associated Press)

Nunes said if Trump's tweets were taken literally, then "clearly the president was wrong."

In an excerpt from a Fox News interview, which will be broadcast in full Wednesday night, Trump suggested that there's more information about the wiretaps.

"I think you're going to find some very interesting items coming to the forefront over the next two weeks," Trump said.

He also said that "wiretap covers a lot of different things."

Questions about Russia

U.S. intelligence agencies have concluded that Russia conducted cyberattacks on Democrats in an effort to influence the 2016 U.S. election on Trump's behalf. Russia has denied this.

At the same time, Trump has been dogged by allegations that his advisers and associates had ties to Russian officials. Trump fired his national security adviser, Michael Flynn, last month after he failed to disclose contacts with Russia's ambassador before Trump took office on Jan. 20.

Comey and Admiral Mike Rogers, the head of the National Security Agency, have agreed to testify before the House committee on issues related to Russia's role in the election.

Schiff and Nunes said they sent a letter asking Comey, Rogers and CIA Director Mike Pompeo to provide by Friday information on leaks of classified information, including names of any Americans who may have been picked up during routine intelligence collection. Noting that this was how Flynn's name surfaced, Nunes said they were concerned about the unmasking of Americans for political purposes.

FBI briefing expected 

Schiff said he expected the Federal Bureau of Investigation to co-operate and was "prepared to support the use of the coercive process" if they did not, referring to the issuing of subpoenas.

In Richmond, Va., Attorney General Jeff Sessions told reporters that he never gave Trump any reason to believe he was wiretapped by the previous administration, according to a transcript provided by CBS News.

Later on Wednesday, Comey was expected to brief leaders of the Senate judiciary committee about any FBI investigation of Russian election-related activities, according to media reports citing the committee's Republican chairman, Charles Grassley. Grassley had threatened to hold up Trump's nominee for deputy attorney general, Rod Rosenstein, until Comey briefed them.

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Senator Lindsey Graham of South Carolina has expressed his frustration at both Trump for his ill-advised tweet, and the FBI for its silence. (J. Scott Applewhite/The Associated Press)

While White House spokesperson Sean Spicer said on Tuesday that Trump was "extremely confident" the Justice Department would produce evidence to support the wiretap assertion, a number of Trump's fellow Republicans in Congress remained unconvinced.

Senator Lindsey Graham, Republican from South Carolina, said he had no reason to believe a judge ever issued a warrant, which would have reflected there was probable cause that the Trump campaign was colluding with the Russians.

'The more suspicious I get'

If it is not true, he said, the FBI should confirm that.

"The longer it takes to answer that question, the more suspicious I get," Graham said on MSNBC.

Graham and Democratic colleague Sheldon Whitehouse from Rhode Island, both judiciary committee members, asked Comey two weeks ago to provide information on Russian activities and the alleged wiretapping by Wednesday. Graham said he was prepared to issue subpoenas for that information.

"Congress is going to flex its muscles," Graham said on CNN.

James Clapper, the director of national intelligence under Obama, has told NBC he knew of no warrant to wiretap Trump Tower.

Congressional Democrats want a special prosecutor or nonpartisan select committee to investigate any Russian ties.

But Trump's fellow Republicans, who control majorities in both the Senate and House of Representatives, say the probes by congressional committees are sufficient.

Tim Kaine, Democratic senator from Virginia and a member of the foreign relations committee, said he believed Comey will say the FBI was "looking at" Russia connections to Trump.

"We have to understand everything about the ties between Russia, the Trump campaign, the Trump transition, the Trump administration," said Kaine, who was the Democratic vice-presidential candidate in the 2016 election.

"Especially the degree to which Russia tried to invade the American election, because we have to protect future elections."