WASHINGTON — Representative Devin Nunes, an outspoken defender of President Trump in his impeachment hearings, said on Sunday that reports that he played a role in the effort to dig up damaging information on former Vice President Joseph R. Biden Jr. in Ukraine were part of a criminal campaign against him by a “totally corrupt” news media.

The reports, including most prominently one from CNN on Friday, nonetheless put Mr. Nunes, the top Republican on the House Intelligence Committee, in the middle of a new controversy at a key moment in the inquiry into Mr. Trump’s pressure campaign on Ukraine.

According to the reports, Lev Parnas, who worked on the Ukraine pressure campaign with Rudolph W. Giuliani, the president’s personal lawyer, said through his lawyer that he had learned of a meeting in Vienna in 2018 between Mr. Nunes and Viktor Shokin, the former Ukrainian prosecutor general who has emerged as a key figure in Republican attacks on Mr. Biden and his son Hunter Biden. Mr. Parnas, who has been indicted on campaign finance charges in Manhattan, learned of the meeting from Mr. Shokin, his lawyer said.

Neither Mr. Shokin nor Mr. Nunes has confirmed that the meeting happened, and Mr. Parnas has no direct knowledge of what may have been discussed. But Mr. Shokin had falsely suggested that he was ousted under pressure from Vice President Biden because he was investigating a company where Hunter Biden was a director, and he has subsequently made claims about the United States ambassador to Ukraine, Marie L. Yovanovitch, aligning him with Mr. Trump.

Mr. Nunes on Sunday refused to directly answer whether he met with Mr. Shokin, instead replying that he wanted to offer a rebuttal but could not yet do so, citing the news media’s “criminal activity” in “listening to somebody who’s been indicted.”

“Everybody’s going to know all the facts, but I think you can understand, I can’t compete by trying to debate this out with the public media when 90 percent of the media are totally corrupt,” he said on Fox News’s “Sunday Morning Futures.”

Mr. Nunes later added after an extended back and forth that the reports were “demonstrably false” and he would “get to all the facts” after he filed a lawsuit against CNN.

Mr. Parnas also met personally with one of Mr. Nunes’s aides at the Trump International Hotel in Washington, according to Mr. Parnas’s lawyer, Joseph A. Bondy. The aide joined at least one meeting there to discuss the effort to collect negative information on the Bidens, Mr. Bondy said. Mr. Nunes did not address that report on Sunday.

Representative Adam B. Schiff of California, the chairman of the Intelligence Committee, declined on Sunday to say whether he found the reports credible or whether the House should investigate. But he noted that they could potentially prompt an ethics investigation.

“If he was on a taxpayer-funded codel — and I say ‘if’ — seeking dirt on a potential Democratic candidate for president, Joe Biden, that will be an ethics matter,” Mr. Schiff said, using an abbreviation for congressional delegation. “That is not before our committee.”

Mr. Giuliani rejected his former associate’s claim on Saturday, telling Fox News that he did not believe Mr. Nunes met with Mr. Shokin.

“Poor Lev, I don’t know what he’s doing to himself,” Mr. Giuliani said. “I think, once again, that’s going to be shown to be provably untrue. This is the first I would have heard of that. I think I would have heard of it, if it happened.”

Still, Mr. Giuliani argued that it “would have been” Mr. Nunes’s “duty” to meet with Mr. Shokin to unearth relevant evidence of corruption in Ukraine.

Mr. Nunes has previously shown a willingness to jump into the fray of investigations to defend Mr. Trump. As the chairman of the Intelligence Committee, Mr. Nunes in 2017 stepped aside from his panel’s investigation into Russia’s efforts to disrupt the 2016 presidential election after the House Ethics Committee opened an investigation into whether he disclosed classified information to the White House.

He was eventually cleared of wrongdoing in that matter. But he continued to be dogged by criticism that he was too close to Mr. Trump to run an impartial investigation, following reports that a claim that Mr. Nunes trumpeted — that the president or his closest associates may have been swept up in foreign surveillance by American spy agencies — was actually fed to him by White House officials.

A former committee staff member for Mr. Nunes, Kashyap Patel, who played a key role in helping Republicans try to undermine the Russia investigation, also has emerged as a figure in the Ukraine matter in recent months. Now an aide on the National Security Council, House impeachment investigators are scrutinizing any role he played in the shadow foreign policy Mr. Trump was conducting as he pushed the Ukrainian government to announce investigations into his political rivals.

Mr. Patel has responded to some of the reports about his role shepherding the administration’s Ukraine policy by joining a defamation lawsuit against a handful of news organizations led by Mr. Nunes, his former boss.

Republicans have also begun their own investigation into the Bidens. Last week, Senator Lindsey Graham, the South Carolina Republican who heads the Judiciary Committee, requested documents from the State Department related to the former vice president’s communications with Ukrainian officials.

Ben Protess and Chris Cameron contributed reporting.