The top Democrat on the House Intelligence Committee rebuked the panel's Republican chairman Wednesday, saying his actions earlier in the day cast doubts on whether the committee can independently investigate alleged Russian meddling in the 2016 election.

Rep. Devin Nunes, R-Calif., held a news conference Wednesday to announce that he had seen intelligence reports showing that U.S. intelligence officials "incidentally collected" information related to Trump transition officials during "normal" foreign surveillance activities unrelated to the investigation into Moscow's role in the election. The Republican congressman then briefed President Donald Trump on the findings, which he said came from an unidentified source. He did not detail whose communications were intercepted and said it "appears to be all legally collected foreign intelligence."

Ranking member Adam Schiff, D-Calif., slammed Nunes' conduct, saying he has still not shared the reports with the committee, making it "impossible" to evaluate the purported communications caught up in surveillance. He argued that Nunes' decision to brief Trump before the intelligence panel makes the case for an independent commission on Russia stronger.

"The chairman will need to decide whether he is the chairman of an independent investigation into conduct, which includes allegations of potential coordination between the Trump campaign and the Russians, or he is going to act as a surrogate of the White House. Because he cannot do both," Schiff told reporters at a separate news conference.

Nunes' announcement came just two days after FBI Director James Comey confirmed that the agency is investigating alleged Russian interference in the election as part of its counterintelligence activities. That probe includes potential ties between Trump officials and Russia.

Comey and the top bipartisan members of the Senate and House intelligence committees have said they see no evidence to back Trump's explosive claim that the Obama administration wiretapped him ahead of the election. Panels in both the House and Senate are investigating Moscow's actions, and the White House asked them to include wiretapping allegations in the broader probe.

Later on Wednesday, when asked on MSNBC's "Meet the Press Daily" if he has seen direct evidence of collusion, Schiff said that there is "evidence that is not circumstantial and is very much worthy of investigation." He declined to go into specifics.

In his press briefing, Schiff contended that Nunes' remarks "throw great doubt" on the House panel's ability to conduct a proper investigation.

"This is deeply troubling along many levels," Schiff said. He stressed that if U.S. citizens' communications were picked up in "incidental" collection, then they were not targets of surveillance themselves.

Schiff later told MSNBC his confidence in Nunes has been "severely shaken" in the wake of Wednesday's events. He said the two will "have a heart-to-heart in the days to come about how we go forward."

Both Schiff and Nunes reiterated they have still seen no evidence to back Trump's tweeted wiretapping accusation. Comey said the same on behalf of himself and the Department of Justice on Monday.

Still, Trump said Wednesday he felt "somewhat" vindicated by Nunes' briefing.

Schiff said, "The wrecking ball of this allegation has just claimed another victim, which is this committee."

Nunes told reporters earlier Wednesday that "additional names" of Trump transition officials were "unmasked," or revealed improperly by the intelligence community, without saying who those people are. Nunes said the unmasking "really bothered" him.

Nunes added that the revealing of names "goes beyond" former National Security Advisor Michael Flynn. Information released about Flynn's calls with the Russian ambassador to the U.S. contributed to his resignation last month.

Schiff said in a statement Wednesday that "most of the names in the intercepted communications were in fact masked."

— CNBC's Christine Wang contributed to this report.