FBI and Justice Department officials gathered with senior lawmakers for two unusual classified briefings related to special counsel Robert Mueller’s Russia investigation, amid rising tensions over President Trump’s accusation that U.S. intelligence spied on his 2016 campaign.

Over the past week Mr. Trump has increased calls for the Justice Department to investigate whether the FBI’s use of a secret informant amounts to what he calls a “deep state” conspiracy of Washington’s elite against him.

Democrats and law enforcement officials have dismissed the charges as an effort to erode faith in Mr. Mueller’s investigation into whether the Trump campaign colluded with Russia and obstructed justice. They have also blasted the Mr. Trump for seemingly outing a valuable intelligence source.

On Thursday, both sides clashed publicly as senior Justice Department officials finally bowed to White House demands for more information and provided a classified briefing to key congressional leaders on the alleged mole at the center of the storm, Stefan Halper, a 73-year-old professor emeritus of Cambridge University who did work for the Nixon, Ford and Reagan administrations.

According to recent reports, Mr. Halper set up meetings in the summer and fall of 2016 to discuss foreign policy with Trump campaign aids Carter Page, Sam Clovis and George Papadopoulos, all of whom have become linked to the Russia collusion probe. Simultaneously, Mr. Halper appeared to be working as an FBI informant for the bureau’s counter-intelligence probe into Russian efforts to meddle in the 2016 election.

Early Thursday, Mr. Trump tweeted that the FBI had finally been caught in a “major SPY scandal” and claimed that former Director of National Intelligence James Clapper had admitted to spying on his campaign.

The incident is “starting to look like one of the biggest political scandals in U.S. history,” Mr. Trump tweeted.

House Speaker Paul D. Ryan, Wisconsin Republican, defended the House Intelligence committee’s struggle with the DOJ and FBI to unearth information on Mr. Halper. House Intelligence Committee Chairman Devin Nunes, California Republican, has led that push by issuing a subpoena for relevant material and threatening top DOJ officials with possible contempt of Congress charges for refusing to comply.

“Inherent in the committee’s work is the responsibility to ask tough questions of the executive branch,” Mr. Ryan said Thursday in a statement. “That is why we have insisted and will continue to insist on Congress’s constitutional right to information necessary for the conduct of oversight.”

White House Chief of Staff John F. Kelly and White House lawyer Emmet Flood briefly attended Thursday’s classified meeting. White House press secretary Sarah Huckabee Sanders said they attended a short part of the briefing to convey “the president’s understanding of the need to protect human intelligence services and the importance of communication between the branches of government.”

Democrats, however, countered that what Mr. Trump has dubbed “Spygate” was nothing but empty allegations that “failed to follow appropriate procedures and protocols.”

“Nothing we heard today has changed our view that there is no evidence to support any allegation that the FBI or any intelligence agency placed a ‘spy’ in the Trump campaign, or otherwise failed to follow appropriate procedures and protocols,” said Senate Democratic Leader Charles E. Schumer, House Democratic Leader Nancy Pelosi, Senate Intelligence committee ranking member Mark Warner and House Intelligence committee ranking member Adam Schiff in a joint statement.

On Twitter, Mr. Warner added, “For the record, the president’s chief of staff and his attorney in an ongoing criminal investigation into the president’s campaign have no business showing up to a classified intelligence briefing.”

The first meeting, held at the Justice Department, was attended by Director of National Intelligence Dan Coats; FBI Director Christopher A. Wray; Deputy Attorney General Rod Rosenstein; Mr. Kelly; Mr. Nunes; Mr. Schiff; and House Oversight and Government Reform Chairman Trey Gowdy.

The second meeting occurred at the Capitol and included lawmakers from the bipartisan “Gang of Eight” which includes the top Republicans and Democrats in each chamber and the top Republicans and Democrats from the House and Senate intelligence panels.

That briefing came about because Democrats objected to the Justice Department and White House initially agreeing to only brief Mr. Nunes and Mr. Gowdy.

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